I run 3 screens, of different sizes (and 1 of those in Portrait).
That does not cause problems by itself (other than losing a Windows location, now and then

)
I don't use Fullscreen.
IIRC Calibre scans your
Systems fonts
for availability upon startup.
You may have junk in that folder that does not belong or a corrupt font. .fon and .ttf are all that belong there