It is working now.The pmxcfs database is malformed!
You will have to restore it from a backup!
If you do not have a backup your last resort will be trying recover the database (e.g. https://stackoverflow.com/questions/18259692/how-to-recover-a-corrupt-sqlite3-database/57872238#57872238)
Anyway make sure to do this without thepve-cluster.service
running
Thank you for your great help.
Merry X'mas.