Backup Error - Fleece dataset already exists

[gfngfn256]

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.