I was able to get this working on windows. I'm not certain, but think the culprit was a failed windows update. I'm using windows 11, but while poking around affinity config files, I noticed a property for the .net version. I remembered that I had a repeated failed windows update for .net. I had to delete the update cache and run them again. After they completed, the file saving went back to normal for all affinity programs.
To check if this is your issue, open the start menu, type "check for updates", and hit enter. if you have a failed update either there on for "Cumulative Update Preview for .NET Framework 3.5 and 4.8..." you may be in the same boat.
Here are the possible steps to getting repeated failed updates working. https://windows101tricks.com/windows-11-update-failed/
