Ok, so I carried out some (more) tests to prevent me from having this “issue” again.
It seems I have to admit that it was my fault: It is impossible to restore a snapshot to a directory I do not have writing permission to (obviously).
As a default user I mounted an exFAT partition to a freshly created directory unter /mnt using sudo mount /dev/sdd1 /mnt/newDir. This results in the partition being mounted in the specified directory with access permissions set to drwxr-xr-x. When performing the same steps with a ntfs drive this results in access permissions set to drwxrwxrwx. The reason is surely that ntfs is mounted as fuse filesystem while exFAT is natively supported by the kernel (for some time past). Because I used ntfs before I was not aware of that difference in behavior and it did not occur in my tests when first time using restic and trying to restore files.
T.l.d.r.: For me the solution was (/would have been) to make sure I have sufficient writing permissions.
I will open up a feature request related to this topic: The reason I did not found the issue myself was that the error messages indicating the real problem where dumped out of my shell by the thousands of lines of consequential error messages. I think that could be improved.
@MichaelEischer Thanks again, your post has shown me the direction to track down the issue! Will you as a moderator maybe mark this post as answer?
Sorry for bothering you all.