We upgraded VSC from 2.1.2 to 4.1 and our backups stopped working. All filers disappeared from Backup and Recovery. I can see them in Monitoring and Host Configuration and we even still have our backup jobs configured. But when I try to add all filers again in Setup section I get: "Could not log onto the storage system with the provided credentials".
I'm gonna answer my question myself, just in case someone run into the same issue. It's turned out that 4.1 is incompatible with Data ONTAP 7.3.2. The oldest ONTAP VSC 4.1 will work with is 7.3.4. So workaround in our case was to revert back to 2.1.1. But make sure to save all metadata before deleting 4.1. Otherwise you'll lose your configuration.