no clean server shutdown possible and
[ftbbackups2_QuartzSchedulerThread/DEBUG] [net.creeperhost.ftbbackups.repack.org.quartz.core.QuartzSchedulerThread/]: batch acquisition of 0 triggers

happening endlessly after shutdown command started.

what is that about?
https://mclo.gs/9CIh7Pe
last lines showing QuartzSchedulerThread

I just set up a 1.20.1 server and have been trying to figure this out as well.

It appears to be related to one of these 2 values. I changed both and the server no longer hangs on shutdown.

"minimum_free_space": 4096,
"free_space_if_needed": true,

I have not been able to reproduce this issue.
The debug messages from Quartz don't necessarily mean that Quartz has anything to do with this.
The Quartz threads are configured to shut down when the main server thread shuts down.

From the logs, it looks like the server gets to the "Saving worlds" step, but never actually finishes saving the words.
Which would suggests that something is hanging during the world save process.
If this is reliably reproducible, then please check if the issue can be reproduced without backups installed.

"minimum_free_space" and "free_space_if_needed" should not have any effect unless the server is out of drive space.