Most likely the files are indeed encrypted by EFS, so you need to use the service account that actually encrypted them. Another possible reason is that the files were generated on Mac/Linux.

You can always send us the diagnostic logs for investigation of issues like that, but currently it looks like a problem that will be fixed in version 6.1.

Also, please check out this thread: https://forum.cloudberrylab.com/discussion/852/cloudberry-backup-now-warns-about-efs-encrypted-files-instead-of-backing-them-up