Yeah! Removing all the backups etc.20xx-xx-xx made with an other user than oneadmin (like root) from previous upgrades fixed everything. onehost sync is now working perfectly, monitoring is working and hosts are not in ERROR state anymore.
I guess this part needs some improvement to raise better error messages in the logs. Because even in debug level I had no clue that this could be such a simple problem.
Anyway, thank you all for your help. I’m now enjoying OpenNebula 5.12.
best regards,
Edouard