Restoring backups from SD
In order to successfully select a backup directory via the DocumentProvider storage, you should not navigate to the chosen folder immediately after choosing the abovementioned storage.

What you should do, is to navigate only to the root directory of the external SD card and then tap on Select. Next, after the Access granted popup appears, use Titanium Backup's built-in file explorer to navigate to the chosen backup directory, and then tap on Use the current folder.