Thank you for yor answer @underwoodblog!
It seems, i found the solution by myself. I deactived the recycle bin option for the backup volume and now restic doesn’t complain anymore. Will do some more testing if this really did fix it.
For the SFTP rights, i checked and even when i connected via sftp user@nas i could see and remove the files. From my understanding, restic should have used the same mechanics.
Do you guys think this is a bug?