Ok sorry, after simply ignoring all the error messages and giving AE some time to cool down it's now—for whatever reason—somehow working again. But I'd be still thankful for any advice to avoid this problem in future and fix the problem in a correct way (not just clicking the "ok"-button till it stops poppig up…).
We can't know. You have not offered up any useful info like actual system info, what fonts, language or even what specific text you use in your project. And for what it's worth: OSX' font validation is useless. Even if a font renders elsewhere, this means nothing for AE. Sometimes even using a specifc character can trigger odd behaviors when the glyph contains bad contours or some Unicode/ OpenType stuff attempts to kick in, a lot of which AE doesn't support.
Well then here they are!
System: macOS Sierra 10.12.2, After Effects CC 2017
Font, which is assumably making the trouble: "TT-Firs" installed in fact in OpenType-Format .otf
Language: German, but I didn't use any "Umlaute" (ä / ü / ö) or otherwise special characters
While working with the somehow crashed/somehow working AE-project I came across this curiosity:
In the Text-Panel there are two "TT-Firs" fonts to choose: "TT Firs Horizontal skalieren (scaled horizontally)" and "TT-Firs (OT1)". Opening up a new textfield with "TT Firs Horizontal skalieren" selected will make the error pop up again like crazy. OT1 is working properly, but will set the letters underneath each other. So I guess the problem is somehow caused by those two options—but how and why?
Sorry for your troubles, but happy you found the culprit.
So I guess the problem is somehow caused by those two options—but how and why?
I've never heard of that behavior before. I think we can assume that these fonts are corrupt or are exhibiting some of the behavior that Mylenium was speaking about. You might try reinstalling them.