Backup Error - Fleece dataset already exists

How can this be avoided in future?
Shutdown any random VM to solving this is not an option.
As fiona said above:
removing the fleecing image is enough.

So that's all you've got to do. If you meant by "avoided in future" as not getting that error in the first place (Backup Error - Fleece dataset already exists), I can't really help you. It looks like from the above reported bug that a patch is available.
 
Same issue again yesterday.
This is not really "Enterprise" or "High availability".

There must be an option to manually remove the stalled flecing-image without stopping the related VMs.
 
Same issue again yesterday.
This is not really "Enterprise" or "High availability".

There must be an option to manually remove the stalled flecing-image without stopping the related VMs.
Can you rename the fleece file with the VM on?