Can't save any Keynote-files because I lack permission (even though I'm admin)?!

O. M. G. I found a solution. After talking to Apple Support twice, deleting and re-installing Keynote, Pages and Numbers, and innumerable re-starts working through my login items, one at the time, I found an old post from 2019 with a similar problem – and an actual solution. It's about the number of activated fonts. I would have never found this out myself (neither did any of the Apple supporters I talked to).

I'm not certain about the technical details, but it's something about Apple's sandbox and Appstore apps – there are 2.000 slots (don't know why or in what context), and for some reason, activated fonts take up those slots (independent of which font management app you use). So having more than 2.000 fonts activated 'blocks the slots', and causes these issues (Keynote, Pages and Numbers being unable to save documents while claiming lack of permission, and not being able to insert media of any kind).

I deactivated a number of fonts, and lo and behold, it fixed the problem. Crazy! I really hope this helps others in a similar solution, and can be found easier than the post I originally found the solution in.

Wow. A problem that's 100% within Apple's sphere, and after 36 hours, and a call to Apple Support, I'm NO closer to a solution – this is rather scary. I've been an Apple evangelist for 25 years, even though quality and service have declined ever since.

Back when I started to use macs (in my case 1998, on iOS 9), everything was noticeably better than on the Windows side (not really the case anymore, sorry). Mac supporters knew what they were talking about. Apple-folk were dedicated nerds who knew more about the platform than anyone else (I myself held lectures to the danish Apple user community on "iTunes – what is it, and what can it do?"

Today, as I mentioned, you call Apple support (supposedly the ones who know most about the platform), and what do you get? A confused student whose best bet is to update your OS (which did not help at all) – in other words, they guess based on predefined scripts, as opposed to actually know what they're talkling about. A sad state of affairs.

Still, I would really like to stick with Keynote, and not have to resort to f'ing Powerpoint. And I need to do so until tomorrow. Any help? Please? Apple? I gave you thousands of dollars for more than 20 years. Anything?

Minor update – I tried a suggested solution from an old thread: to restart in Safe mode, then to create files from Keynote, Pages and Numbers (problems apply to all of them). After another restart, this was supposed to fix the problem. Which it didn't, unfortunately.

While I'm in Safe mode, everything was fine. Was able to save files, and to insert media. But as soon as I went back to a 'normal' restart, the problems were back.