Sceduled Backup doesn't work anymore

Hi folks, I used to scedule a backup once a day from the readynas drive to a the external usb3 drive. This worked for years. Without any reason, the scedule doesn't start any more. Manual starting works fine, but the scedule doesn't start anything. The logs stay empty. Everything is connected right and working, as manual starting works. System time is correct and synchronized. The system just ignores the scedule. I triple checked all, can't find the solution. The backup job is a simple complete volume job. The destination needs no password, it is just as simple as it could be. Any ideas? Thanks for help.