Since fonts seem to cause the problem try the following first:
- Stop or disable the
Windows Presentation Foundation Font Cache service, if it's running
- Delete the font cache file
C:\Windows\System32\FNTCACHE.DAT and restart your system
BTW, did you check the Qt version? It's 4.7.4 on my machine:
If you have the same Qt version and the font cache changes had no effect, your may want to opt for the "nuclear option."
I.e. back up your Calibre library and any books or other files that you might have saved in the Sigi and Calibre program folders.
Then completely uninstall both Calibre and Sigil, delete any leftover files in the program folders, clean the registry and reinstall one at a time and test whether your problem occurs in one of the apps or both.