I just tried the installer on a FP3+ tonight, and it worked
Before my install with the UBports-installer, I had tried to install using this guide: https://forums.ubports.com/topic/5373/fairphone-3-fp3-port
That was not successful, and left the phone without any working OS. In this state, I tried the new Ubports-installer, and all my troubles was gone ! Great work ! Thanks !

It worked! I could reflash, and all my data was still there inclusive Anbox

Reply to myselves and all others in same situation:

• open adb shell to the phone with the new installation
• Go to the Authenticator NG folder: cd ~/.config/authenticator-ng.dobey
• Rename the current config file, in case you want it back later: mv authenticator.conf [newname]
• exit ADB
• Push your saved authenticator.conf to the reight location on the new installation: adb push [local path/authenticator.conf] /home/phablet/.config/authenticator-ng.dobey