Hi Jakob,
After reading around the docs Patrick linked, I noticed that both Feature.enabled?(:execute_batched_migrations_on_schedule) and Feature.enabled?(:optimize_batched_migrations) returned false.
I just checked these settings for me and both returned true. I manually restarted the failed backgound migrations and all except one finished successfully (see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1015301).
Best, Maximilian
