Good luck! Honestly it can be like playing Whack-A-Mole if you have a busy database :D.

No, if another transaction log backup is taken...

You say there are no maintenance jobs, but are there any SQL agent jobs? The backups may be being initiated on another server/instance, too.

Have you tried doing a manual checkpoint on that database?

Can you check the msdb.dbo.backupset history to see if it can shed more light on which account initiated the backup? There's a user_name column.

Also, what format is the...

The only way to do it is to remove it and create it again. Alternatively you could create a DNS alias with the name you want and point it at...

What Leo Miller is referring to is correct. What you really need when you can get some downtime is fail over to the other side, and drop and recreate the...

Sorry for your loss, Jeff. What a tough time many people are having right now.

It's because differential backups write to the database (to update the differential bitmap), and that's not possible in a secondary replica because it is read-only.

Lynn, I'm so sorry for your loss.

Can you give us some more detail please. Where did you restart the service?

It's a known issue/expected behaviour. If you don't need synchronous commit mode*, you can mitigate some of the issues by switching to asynchronous mode.

 suspect this is...

I think adding your jurisdiction/location/retention requirements and any other regulatory considerations might help others to know what to recommend to you. Reading between the lines I think you are probably...