I did the recommended backup via clonezilla --> worked very well including testing of the image
Did the upgrade --> worked well
First thing I encountered was the MySQL Database could not be accessed. After resetting the root/omvadmin password this worked as well. Actually, I don´t really know what happened exactly or what made it work in the end.
I also had the "rrdcached plugin could not connect to..." error in syslog. After enabling and disabling the Monitoring this error disappeared.
So far, I'm really happy with the upgrade.