I had to install 18.04, then install Cloudron 5.5.0, then restore the backed up cloudron, then upgrade it to 6.1.2, then make a backup.
Then destroy the 18.04 and install 20.04 instead, then restore the 6.1.2 backup....puhh. Done, working!
(Could not restore a 26GB Nextcload app, but no problem, had the files backed up locally)
Guess this thread can be marked as solved?