Known issue. Will be fixed in the 12.1 update in Occtober. For the time being, remove any fonts that have special chracters in their name like accents, umlauts, trademark signs and so on.
Mylenium is correct. The specific issue is because of how the name of the font is processed within After Effects, and "special" characters like those that he mentions trigger the problem. Also likely culprits are fonts with names that use characters that are not in the character set of the language in which you're running your operating system---e.g., a Japanese font on a system running software in English.
This is fixed in the version of After Effects coming out in a few weeks.