Here's what seems to be an interesting little buglet and/or incompatibility.
The workaround is simple and I hope that the information below will save
others from having to discover it for themselves.
My home box is on the web only very intermittently. It finished a LL test
several days ago and did not have another exponent ready to test, so I set
it going on some ECM factoring.
Just now, I started the dial-up connection and tried to upload the LL result
manually, with no discernable effect whatsoever. I then stopped the ECM
curve and tried again. This time, the server was "busy" (Error 23 returned)
and Prime95 V16.4.1 started a new ECM curve; the effect was reproducible. I
know the server was ok, because I was looking at various status pages.
The workaround is to rename the worktodo.ini file in the Prime95 folder and
to re-run the manual communication. My LL result is now on the server and
I've been allocated a new exponent.
Paul