Synology Diskstation Local Backup Does Not Overwrite Changed Files

I've added a timed local backup on my Synology Diskstation DS409. I added a remote shared folder to the station that I want to backup. I do NOT want that the remote system backups TO the Diskstation, I want the Diskstation to GET the contents from the remote system.

However, the backup itself works fine. It started and copied all files from the share to a local folder. Now on the next schedule run, the backup only adds new created files during that period. Old files, that are changed won't get copied. It does not matter if I use incremental feature or not. It neither does overwrite files to a newer state nor keep an incremental version of the changed file.

However, newly created files and folders successfully get added.