how to remove a former backup folder with a 90 VHD file on a networkdrive?

Question

I want to remove this huge Backup file with its folder structure from 2016. It was made by Windows Backup. Later I must have copied to another folder. The folders are now on an Synology NAS, on an shared drive. In the security Tab of the properties of the folder is an unknow account that cannot be remove as well, on any level. It has no name, just an S and a lot of numbers: I suppose the security ID of the account used by Backup. It is not possible to remove, or to take ownership of the file. Any attemp to change rights on whatever level/ folder stops because changes cannot be written on those backup files. Even if I log as the Admin of the Synology Drive it is impossible to change/ write changes to. Grrrrr!