Welcome to the forum @compu
It’s odd that one VM is backing up OK and the other isn’t. Are there any differences besides usage?
I don’t know why an Exchange Server VM would have special needs. Do the VM’s files look similar?
What Are the New File Types in Windows Server 2016 Hyper-V VMs? is one description of suffixes.
VSS Writers and their Services has info allowing vssadmin list writers in VM to see what’s up.
What are the requirements for live backups of a Hyper-V Guest VM? and related links has other info
probably relevant to Duplicati’s VSS use as well (no guarantee), from a specialist backup company.
I think both backups should be using snapshot-policy=required, but I “think” warn you if that’s not so.
Because this uses VSS, Duplicati must be at least an elevated administrator, e.g. SYSTEM service.
You should get a better look at the warnings by watching with About → Show log → Live → Warning
(clicking on lines to expand them). Or you can set up a log-file=<path> to catch warnings and errors.
How many versions does Restore think there are? Did you get a job log? If so, look in Complete log
“BackupListCount” is the version count. You can also view <job> → Show log → Remote’s top lines.
would be a typical reverse-chronological ending with a dlist file listing the files in backup. Got one?
You could also look on the destination using a reverse-chronological sort or a name search for dlist.
If no dlist, then no backup. You could see About → Show log → Stored to look for any other failures.
Backups that don’t complete tend not to leave the usual results log, but may complain to server log.
Duplicati hyper-v has thoughts from another person, but I’m not sure there are any experts available.
Depending on how business-critical these systems are, Duplicati (still in Beta…) might be a poor fit.