In case Windows native encryption (EFS) is used - you might want to follow the measures described on this page.
Please note that instead of SYSTEM you should use your Backup Service account (usually differs from SYSTEM)
https://superuser.com/questions/1014519/how-do-i-let-the-system-account-use-efs-encryption

In case third party (e.g. application internal) encryption is used - the data should be decrypted first.

It is the native Windows EFS. By Backup Service account do you mean the service account do you mean the one specified in the Tools tab in Cloudberry Backup?

Please refer to this blog post on the Backup Operator implementation. Thanks! https://www.msp360.com/resources/blog/support-for-backup-operators-api/

I did figure out the Backup Operator part. Seems like something wasn't right with the EFS implementation, so it's turned off for the moment. That could also be why the backup wasn't working.

EFS Support was added to the latest Windows 7.6 agent. https://help.mspbackups.com/backup/about/efs

You have options to either Keep EFS Encryption or Decrypt EFS files at backup time. The new feature is only available when using the New Backup Format.