Cannot backup VMs due the remaining overlay files

those files are not created by PVE.. are you using some third party backup integration?
 
We are using NAKIVO. Does it mean, those files are created by this 3rd party backup software?
If yes, I will raise the ticket to the NAKIVO support. At the moment, I do not know, if I can delete the files or not and backups are frozen:-(

Petr
 
very likely, yes.