I have experienced significant problems with Virtualmin Backup/Restore when quotas are mismatched between the backup Virtualmin and the restoring Virtualmin.
The situation is that I am moving from one VPS (rosehosting.com) running Virtualmin GPL 3.73 to another VPS (virpus.com) running Virtualmin GPL 3.73. Both installations were completed using the install script. The rosehosting (backup) Virtualmin VPS does not have quotas enabled, and the virpus (restore) Virtualmin VPS does.
The issue also occurs going the reverse direction - from a system with quotas enabled to one without.
This bug renders the backup/restore capability of Virtualmin useless (and dangerous, because one believes one is "safe" if they have backups). A restore attempt always fail with the following message:
Restore failed : setquota: User [username] doesn't exist.
I also don't seem to have the ability to elect to NOT backup or restore quotas.
If you need any more information to help solve this issue just let me know.