The 5.1 and 5.2 preferences are going to be incompatible. As are the databases. So you don’t want these 2 versions cross-pollinating.
5.2 hasn’t been released into public beta yet. So refinements (like segregating the .ini files in the User Directory… or not failing when trying to reading incompatible .ini files… or not trying to automatically update your version 19 5.1.x database schema to the version 20 of the 5.2.x version)
You should have these versions in separate VMs. This alpha version is high risk. And the Beta will be too. (Or it would be release, rather than in beta.)
This is on a fresh install of Manjaro with no other gramps installations. Deleting everything and cloning the repository, preference works fine. On save and then exiting Gramps, when restart, I then get the above error message. The Gramps I’m using for day to day is on another hard drive and OS so no mixing. The reason I’m messing with 5.2 is to be ready for when it comes out in Aug so I can move over to it quickly. 5.2 has features I’ve been wanting for a long time and what I see so far, 5.2 is going to smoke the 5.1 versions.
The Themes addon adds itself to the tabs found in Preferences and the tab names must agree. When I reworked the Preferences file, I also set the Themes for the changed tabs. I have not checked to see if others made more changes that need to be accounted for.