Hello everyone!

Just wanted to give feedback about my problem!

Updated to 4Ext Recovery 1.0.0.5 RC3 prior to installing Supernova Extreme and made a Nandroid Backup without the problem I encountered before!

Did two additional Nandroid Backups after flashing the new ROM also without any problems!

So this thread can be closed :)

Thanks again to everyone for your helpful ideas!

Regards,
Guido