Hello @MarcelR
If you don’t mind, I will join this thread as well.
Thank you for the provided video. According to it, once the file was opened, you immediately tried to change the fonts. Could you please run a test? Create a new file and await a bit. After that try to click Fonts box. As @Nikolas mentioned before, some time is needed for parsing system fonts.
Please let us know the result, if the issue still persists please attach new video file.
Hi @Alexandre, in another DM thread, I already wrote Nikolas that we had waited for hours(!) already (I read on the forum this might solve sometimes). Yesterday, we even waited for 1 night (opened Saturday evening and checked Sunday morning). We spent hours already in all other options we could imagine: different kind of documents (text, spreadsheet, etc.), restarting Mac, uninstall (throw app in bin, delete bin content) and install again. All without success…
Thank you for your quick response, we are checking the situation. I will contact you shortly.
Dear @MarcelR
Please provide us with the fonts list that you’re using on your device. This title might help you to find them: Install and validate fonts in Font Book on Mac – Apple Support (UK)
Hi @Alexandre . How (in which format)? In ‘All fonts’ 548 typefaces are listed and in ‘My fonts’ 209. I can’t export a list of fonts, I only can copy and paste the list in a spreadsheet (CSV), but then it’s a list of ‘urls’ to the font location. Which I don’t want to share in public forum.
What fonts are loaded/used in Onlyoffice: active/inactive? About 95% of fonts are not added by me, it’s fonts installed by Apple and/or installed programs. Can I test by creating a (smart) collection and configure Onlyoffice in a way only that collection is used?
Hi @Alexandre, problem is solved. After resolving duplicate fonts, restarting Mac, resolving duplicate fonts (again, first time it failed?!), restarting Mac again and launching Only Office, fonts are listed in apps. 
Wow! Thank you for your update! We are glad that the issue has been resolved. So far, it’s not clear why duplicate fonts appeared. We will try to take a look at this scenario.
Thank you one more time!