Well, no one had a similar issue... 
What I did:
I had to delete all entries from mdl_backup_courses and choose a closer date to finally get the backups done.
I also changed \core\task\automated_backup_task on https://[mymoodlesite]/admin/tool/task/scheduledtasks.php from every 50min to 15min in order to run it more closely to the scheduled date.
Best!
What I did:
I had to delete all entries from mdl_backup_courses and choose a closer date to finally get the backups done.
I also changed \core\task\automated_backup_task on https://[mymoodlesite]/admin/tool/task/scheduledtasks.php from every 50min to 15min in order to run it more closely to the scheduled date.
Best!