Hello together,
it seems either to activate IO Thread or to change the controller to VirtIO SCSI Single has solved the freezing issue. The affected VMs are running now since five days without any issue. To find the exact solution reason I'll do some more investigations after my return from business trip. But for the moment I'll mark this issue as solved.
Thanks to all for the valuable hints and tips, with a special thanks to Fiona, for the IO Thread / VirtIO SCSI hint!
Thanks and regards,
Jörg