Just a warning to any Mac users. I have upgraded from 5.1.6 to 5.2 on Intel based Mac running Monterey 12.7.3. Used official dmg package. On first run I was asked to allow upgrade of the database which I did. Ended up with an error resulting in empty database. Quite scary initially. Recovered by replacing the “new” database with a copy of 5.1.6 that I wisely took before the upgrade.
I have filed a bug report (#13196) as suggested by the program.
Just discovered that 5.2 creates a compressed .zip copy of the existing database in gramps folder, so no danger of the data loss after all. 
I looked at your report, and if the import of a .gramps backup didn’t work, and a GEDCOM import did, you have data loss now, in your notes. All text styles and links have probably disappeared.
Just for info, when I upgraded from 5.1.6 to 5.2.0 on macos Sonoma 14.2.1 on Apple M1, the database upgrade process appeared to go ok.
I did a similar upgrade for all databases that I have on my laptop, where I already installed 5.2 to test, in LMDE 6 (Debian based), and these upgrades were all OK too.
This is why I’m sure that the problem is triggered by a specific entry in @PeterO 's database, and why I hope that he can attach a .gramps file to his bug report, so that we can reproduce the error.
Using GEDCOM files as a workaround is not a good idea.
I upgraded from 5.1.5 to 5.2.0. I had crashes when i tried to edit certain records.
Went back to 5.1.5 again an everything is working.
I created a ticket (#13192) for this.
I accept that, just need to find the offending entry. Not sure how to proceed since everything looks fine in 5.1.6 including database checks.