Backup files invisible in windows explorer. Are they there at all?

@support I run ROCK on a NUC, without any problems. Until yesterday, I used a USB Stick attached to the NUC to back up my roon database to. Yesterday, I switched to a spare USB HDD. The drive is formatted to ExFAT.

Now, roon backs up just fine (at least that’s what it says), but I can’t see any files in windows explorer. There are no files shown, where in the past were a whole bunch of subdirectories and files.
When I do a right-click on \ROCK\Data\Storage\ (my HDD)\RoonBackups, I get the information that it contains several thousands of folders and files. When I open the folder, I see a folder with a name consisting of numbers and letters and a folder called System volume information. The numbers and letters folder (who should contain the backups, if I’m not mistaken) is empty.

Roon can see my backups when I access them via setting --> backups --> find…, but I can’t say if they are empty or regular backups. I didn’t dare to try and restore one of them.
How can I tell if the backup works fine, and how can I access the files in windows explorer, so that I can sync them to other locations?