It turned out to be a mounting problem. Once I had run through everything once more it worked even if it is a little unstable (MC22 crashed while importing media) but that may be due to the old Raspberry I am using.
For those who are also trying to mount a Synology share on on the Raspberry Pi, this is what eventually worked for me:
Install MC22 on the sd card:
https://yabb.jriver.com/interact/index.php?topic=106027.0Enable NFS on the Synology NAS: file services > NFS > enable
Go to shared folder you would like to mount on Diskstation: Shared folder > Edit > NFS Permissions > Create Hostname/IP: IP address of Pi; Privilege: read only; Squash: map all users to admin; Security: sys and tick all 3 options below (Enable .. Allow connections .. Allow users ..)
Open Terminal on Pi and follow instructions given here:
https://mydnic.be/post/hcp-2-access-synology-shared-folders-from-raspberry-pi