Errors in all HYPERV's registration of snapshot set failed

For clients we seem to ge the same error message

nsrsnap_vss_save:nmm.. registration of snapshot set failed -- nmm .. operation unit failed with error. Error encountered in remote agent for unit id: 256, Errorhandler invoked.

I see from an old community chat that this could be a  something to do with:  1. unsupported filesystem 2. write/protect disc  3. No space on disc   4. Drive not found

Anybody have this problem before ?

Any advice would be most welcome, thank Phil