Always On shared network location
Well it took a while but finaly found it myself. Besides the service account it also requires full access for the user of SSMS. Sounds obvious probably and indeed I thought about it earlier but since the account was already in the list of administrators and this group already had full access to the shared volume I thought it would have been enough. So what was required was just adding the SSMS user/group and service account to the advanced sharing of the SYNC folder.