>From Andrew: ----- l3backend was the only missing package I was aware of. There are various MiKTeX log files available. You need to dig deep to find them. On my system they're at
C:\Users\Andrew\AppData\Local\MiKTeX\2.9\miktex\log See which ones are getting added to when you try to run LyX (sort them by date/time) ----- I am the embodiment of the digital age frogman. Digging deep is what I do. Parse it, hex edit, sed, grep, and awk, compile, decompile, debug, and log... bit@bitbucket MINGW64 ~/AppData/Local/MiKTeX/2.9/miktex/log $ ls -rt * | xargs grep cls kpsewhich.log:2019-07-13 11:32:46,821-0500 INFO kpsewhich - starting with command line: kpsewhich.exe article.cls latex.log:2017-11-15 13:33:53,532-0600 INFO latex - installing package ieeetran triggered by tex\latex\ieeetran\IEEEtran.cls latex.log:2017-11-15 13:34:00,298-0600 INFO latex - installing package a0poster triggered by tex\latex\a0poster\a0poster.cls latex.log:2017-11-15 13:34:03,097-0600 INFO latex - installing package aastex triggered by tex\latex\aastex\aastex6.cls latex.log:2017-11-15 13:34:10,346-0600 INFO latex - installing package achemso triggered by tex\latex\achemso\achemso.cls latex.log:2017-11-15 13:34:28,377-0600 INFO latex - installing package aguplus triggered by tex\latex\aguplus\aguplus.cls latex.log:2017-11-15 13:34:31,743-0600 INFO latex - installing package apa6 triggered by tex\latex\apa6\apa6.cls latex.log:2017-11-15 13:34:41,925-0600 INFO latex - installing package apa triggered by tex\latex\apa\apa.cls latex.log:2017-11-15 13:34:44,424-0600 INFO latex - installing package arabtex triggered by tex\latex\arabtex\arabart.cls latex.log:2017-11-15 13:35:05,887-0600 INFO latex - installing package ctex triggered by tex\latex\ctex\ctexart.cls latex.log:2017-11-15 13:35:16,936-0600 INFO latex - installing package dinbrief triggered by tex\latex\dinbrief\dinbrief.cls latex.log:2017-11-15 13:35:23,718-0600 INFO latex - installing package dtk triggered by tex\latex\dtk\dtk.cls latex.log:2017-11-15 13:35:36,352-0600 INFO latex - installing package elsarticle triggered by tex\latex\elsarticle\elsarticle.cls latex.log:2017-11-15 13:35:39,934-0600 INFO latex - installing package elsevier triggered by tex\latex\elsevier\elsart.cls latex.log:2017-11-15 13:35:43,356-0600 INFO latex - installing package europasscv triggered by tex\latex\europasscv\europasscv.cls latex.log:2017-11-15 13:35:59,061-0600 INFO latex - installing package europecv triggered by tex\latex\europecv\europecv.cls latex.log:2017-11-15 13:36:09,248-0600 INFO latex - installing package extsizes triggered by tex\latex\extsizes\extarticle.cls latex.log:2017-11-15 13:36:11,701-0600 INFO latex - installing package frletter triggered by tex\latex\frletter\frletter.cls latex.log:2017-11-15 13:36:13,764-0600 INFO latex - installing package g-brief triggered by tex\latex\g-brief\g-brief2.cls latex.log:2017-11-15 13:36:16,498-0600 INFO latex - installing package platex triggered by tex\platex\base\jarticle.cls latex.log:2017-11-15 13:36:23,716-0600 INFO latex - installing package jsclasses triggered by tex\platex\jsclasses\jsarticle.cls latex.log:2017-11-15 13:36:34,419-0600 INFO latex - installing package kluwer triggered by tex\latex\kluwer\kluwer.cls latex.log:2017-11-15 13:36:37,278-0600 INFO latex - installing package lettre triggered by tex\latex\lettre\lettre.cls latex.log:2017-11-15 13:36:41,169-0600 INFO latex - installing package tugboat triggered by tex\latex\tugboat\ltugboat.cls latex.log:2017-11-15 13:36:44,872-0600 INFO latex - installing package memoir triggered by tex\latex\memoir\memoir.cls latex.log:2017-11-15 13:36:53,574-0600 INFO latex - installing package moderncv triggered by tex\latex\moderncv\moderncv.cls latex.log:2017-11-15 13:37:11,808-0600 INFO latex - installing package mwcls triggered by tex\latex\mwcls\mwart.cls latex.log:2017-11-15 13:37:11,933-0600 INFO latex - mpm: downloading ftp://ftp.fernuni-hagen.de/pub/mirrors/www.ctan.org/systems/win32/miktex/tm/packages/mwcls.tar.lzma. .. latex.log:2017-11-15 13:37:13,745-0600 INFO latex - mpm: extracting files from mwcls.tar.lzma... latex.log:2017-11-15 13:37:15,823-0600 INFO latex - installing package paper triggered by tex\latex\paper\paper.cls latex.log:2017-11-15 13:37:19,948-0600 INFO latex - installing package powerdot triggered by tex\latex\powerdot\powerdot.cls latex.log:2017-11-15 13:37:27,557-0600 INFO latex - installing package koma-script triggered by tex\latex\koma-script\scrbook.cls latex.log:2017-11-15 13:37:49,087-0600 INFO latex - installing package revtex triggered by tex\latex\revtex\revtex4-1.cls latex.log:2017-11-15 13:38:01,604-0600 INFO latex - installing package revtex4 triggered by tex\latex\revtex4\revtex4.cls latex.log:2017-11-15 13:38:05,573-0600 INFO latex - installing package sciposter triggered by tex\latex\sciposter\sciposter.cls latex.log:2017-11-15 13:38:08,683-0600 INFO latex - installing package seminar triggered by tex\latex\seminar\seminar.cls latex.log:2017-11-15 13:38:12,043-0600 INFO latex - installing package simplecv triggered by tex\latex\simplecv\simplecv.cls latex.log:2017-11-15 13:38:15,669-0600 INFO latex - installing package spie triggered by tex\latex\spie\spie.cls latex.log:2017-11-15 13:38:26,655-0600 INFO latex - installing package tufte-latex triggered by tex\latex\tufte-latex\tufte-book.cls latex.log:2018-11-17 15:44:50,618-0600 INFO latex - installing package acmart triggered by tex\latex\acmart\acmart.cls latex.log:2018-11-17 15:45:01,718-0600 INFO latex - installing package bxjscls triggered by tex\latex\bxjscls\bxjsarticle.cls latex.log:2018-11-17 15:45:03,548-0600 INFO latex - mpm: downloading ftp://ftp.fernuni-hagen.de/pub/mirrors/www.ctan.org/systems/win32/miktex/tm/packages/bxjscls.tar.lzma. .. latex.log:2018-11-17 15:45:10,878-0600 INFO latex - mpm: extracting files from bxjscls.tar.lzma... latex.log:2019-07-13 12:37:49,298-0500 INFO latex - installing package ieeetran triggered by tex/latex/ieeetran\IEEEtran.cls latex.log:2019-07-13 12:37:57,296-0500 INFO latex - installing package a0poster triggered by tex/latex/a0poster\a0poster.cls latex.log:2019-07-13 12:38:02,911-0500 INFO latex - installing package aastex triggered by tex/latex/aastex\aastex62.cls latex.log:2019-07-13 12:38:15,057-0500 INFO latex - installing package achemso triggered by tex/latex/achemso\achemso.cls latex.log:2019-07-13 12:38:34,351-0500 INFO latex - installing package acmart triggered by tex/latex/acmart\acmart.cls latex.log:2019-07-13 12:38:59,777-0500 INFO latex - installing package aguplus triggered by tex/latex/aguplus\aguplus.cls latex.log:2019-07-13 12:39:05,808-0500 INFO latex - installing package apa6 triggered by tex/latex/apa6\apa6.cls latex.log:2019-07-13 12:39:24,403-0500 INFO latex - installing package apa triggered by tex/latex/apa\apa.cls latex.log:2019-07-13 12:39:29,584-0500 INFO latex - installing package arabtex triggered by tex/latex/arabtex\arabart.cls latex.log:2019-07-13 12:39:54,343-0500 INFO latex - installing package bxjscls triggered by tex/latex/bxjscls\bxjsarticle.cls latex.log:2019-07-13 12:39:55,193-0500 INFO latex - mpm: downloading http://mirrors.rit.edu/CTAN/systems/win32/miktex/tm/packages/bxjscls.tar.lzma. .. latex.log:2019-07-13 12:39:58,509-0500 INFO latex - mpm: extracting files from bxjscls.tar.lzma... latex.log:2019-07-13 12:40:15,537-0500 INFO latex - installing package ctex triggered by tex/latex/ctex\ctexart.cls latex.log:2019-07-13 12:40:36,147-0500 INFO latex - installing package dinbrief triggered by tex/latex/dinbrief\dinbrief.cls latex.log:2019-07-13 12:40:47,760-0500 INFO latex - installing package dtk triggered by tex/latex/dtk\dtk.cls latex.log:2019-07-13 12:40:53,609-0500 INFO latex - installing package elsarticle triggered by tex/latex/elsarticle\elsarticle.cls latex.log:2019-07-13 12:41:00,506-0500 INFO latex - installing package elsevier triggered by tex/latex/elsevier\elsart.cls latex.log:2019-07-13 12:41:06,305-0500 INFO latex - installing package europasscv triggered by tex/latex/europasscv\europasscv.cls latex.log:2019-07-13 12:41:26,882-0500 INFO latex - installing package europecv triggered by tex/latex/europecv\europecv.cls latex.log:2019-07-13 12:41:42,716-0500 INFO latex - installing package extsizes triggered by tex/latex/extsizes\extarticle.cls latex.log:2019-07-13 12:41:47,932-0500 INFO latex - installing package frletter triggered by tex/latex/frletter\frletter.cls latex.log:2019-07-13 12:41:52,999-0500 INFO latex - installing package g-brief triggered by tex/latex/g-brief\g-brief2.cls latex.log:2019-07-13 12:43:49,165-0500 INFO latex - installing package g-brief triggered by tex/latex/g-brief\g-brief2.cls latex.log:2019-07-13 12:43:58,365-0500 INFO latex - installing package platex triggered by tex/platex/base\jarticle.cls latex.log:2019-07-13 12:44:14,065-0500 INFO latex - installing package jsclasses triggered by tex/platex/jsclasses\jsarticle.cls latex.log:2019-07-13 12:44:39,599-0500 INFO latex - installing package kluwer triggered by tex/latex/kluwer\kluwer.cls latex.log:2019-07-13 12:44:47,382-0500 INFO latex - installing package lettre triggered by tex/latex/lettre\lettre.cls latex.log:2019-07-13 12:45:01,432-0500 INFO latex - installing package tugboat triggered by tex/latex/tugboat\ltugboat.cls latex.log:2019-07-13 12:45:15,731-0500 INFO latex - installing package memoir triggered by tex/latex/memoir\memoir.cls latex.log:2019-07-13 12:45:43,132-0500 INFO latex - installing package moderncv triggered by tex/latex/moderncv\moderncv.cls latex.log:2019-07-13 12:47:39,798-0500 INFO latex - installing package mwcls triggered by tex/latex/mwcls\mwart.cls latex.log:2019-07-13 12:47:40,614-0500 INFO latex - mpm: downloading http://ctan.mirrors.hoobly.com/systems/win32/miktex/tm/packages/mwcls.tar.lzma. .. latex.log:2019-07-13 12:47:43,198-0500 INFO latex - mpm: extracting files from mwcls.tar.lzma... latex.log:2019-07-13 12:47:47,580-0500 INFO latex - installing package paper triggered by tex/latex/paper\paper.cls latex.log:2019-07-13 12:47:54,715-0500 INFO latex - installing package powerdot triggered by tex/latex/powerdot\powerdot.cls latex.log:2019-07-13 12:48:15,131-0500 INFO latex - installing package koma-script triggered by tex/latex/koma-script\scrbook.cls latex.log:2019-07-13 12:49:49,614-0500 INFO latex - installing package revtex triggered by tex/latex/revtex\revtex4-1.cls latex.log:2019-07-13 12:50:19,547-0500 INFO latex - installing package revtex4 triggered by tex/latex/revtex4\revtex4.cls latex.log:2019-07-13 12:50:32,664-0500 INFO latex - installing package sciposter triggered by tex/latex/sciposter\sciposter.cls latex.log:2019-07-13 12:50:38,981-0500 INFO latex - installing package seminar triggered by tex/latex/seminar\seminar.cls latex.log:2019-07-13 12:50:45,731-0500 INFO latex - installing package simplecv triggered by tex/latex/simplecv\simplecv.cls latex.log:2019-07-13 12:50:51,947-0500 INFO latex - installing package spie triggered by tex/latex/spie\spie.cls latex.log:2019-07-13 12:50:59,530-0500 INFO latex - installing package tufte-latex triggered by tex/latex/tufte-latex\tufte-book.cls but as I just recently posted I reinstalled everything and I am experiencing some very strange behavior where LyX reports Unavailable cls, but generates a pdf document regardless.
