Mailing the story:
GIMP 3.0 RC2 Added New public API, Initial AppImage & Fixed 2.10 Migration
The new GIMP 3.0 RC2 fixed the issue migrating user’s 2.10 settings to GIMP 3.0. However, if you already used 3.0 RC1, then you need to delete those configurations first (backup of course), as otherwise RC2 won’t try to import the 2.10 preferences.