Just a suggestion. Open it up in Notepad or other
text editor and see if you can spot some major
difference in the HTML source code. If you don't
have enough experience to spot bad HTML code
visually, you can use a good MHT file that
works and compare it to one from that account
that is giving you problems.

Thanks for the idea - but all of my tested *.mht(ml) files are ok. (They can
be opened at other user profiles, or at other PCs.) No problem of specific
content. Even empty files are affected.