Actually, I found out what the problem was.
For some reason the Arial Family font was turned off in the Font Book. It seems like fm 13 still manages to use it, while FM 14 couldn't.
Everything went back to normal as soon as a enabled the Arial font again.
since this theme isn't on the deprecated list, consider it as a bug...
-> issue report: forums.filemaker.com
I don't have an english system here - I checked some themes: They are Arial on 14 (but I probably missed the correct theme)