@Christoph-Hart
Yes, you're quite right, that probably is the real cause.
Still a bit weird, since an older install worked as should.
Although it was the one with an .pkg, or was it .dmg, installer from the website,
so that might have to do with it as well.
And as the admin and the sole user of the machine, the writing rights should've been ok.

OSX Tahoe 26.5.2 M2 HISE 3deb6f2a

Had the same issues just today after few updates from much older version.
The cog wheel settings not saving at all and snippet + document bowsers not working.
Did few rebuilds, but the solution is to create a new empty HISE folder to
'/Users/yourUserName/Library/Application Support/HISE' and start HISE again.
After putting in the settings again, it will populate the folder with the necessary files.

Why the folder has vanished is anyones guess, the easiest solution is to blame disc cleaning and maintenance softwares if you use one and you probably wouldn't be far off.