This I got: $ kpsewhich scrartcl.cls /usr/share/texlive/texmf-dist/tex/latex/koma-script/scrartcl.cls
I did Tools / Reconfigure, I think that solved most of the problems. I can now open my colleagues document again without errors, and I get the correct pdf-output. Fixed! I still see a whole bunch of greyed-out document classes, but I don't care for now, I'll try to fix that later. Lyx works again! Thanks to you all. Ygo.
