Hi Laurin,
I had this issue before and remember a few things I needed to check to solve it. But first, your device must be running software version 2.7.0 or higher. If that is the case then try taking out the SSD, close the mapping UI and open File Manager on device's Ubuntu. Check if there are any folders under /srv folder. Just remove any folder you find in /srv. That solved it for me and you can put your SSD back into the device.
If however the /srv folder was already empty, I would put back the SSD and try to mount it from the "Disks" application.
Good luck!