There are two kinds of AlwaysOn technology. One is AlwaysOn FCIs, and one is AlwaysOn Availability Groups. The answer to your question largely depends on which one you mean. It...

In addition to the questions above, have you got trace flag 1448 enabled? 

http://blogs.microsoft.co.il/yaniv_etrogi/2017/07/14/trace-flag-1448-replication-with-always-on-availability-groups/

Check here to see which downloads you need: https://sqlserverupdates.com/

I'm concerned about the urgency of this request though; if this is a Production change and you haven't done...

You should be able to do a one-off shrink of the transaction log without needing to change the database to Simple recovery model first. I would caution against shrinking it...

What's the problem you're trying to solve here? The reason I ask is that it can be a bad idea to start moving this kind of data around if you're...

Thanks. My guess is that the database must have failed over to that server for the period covering those backups.

When you say it's a standby server, what do you mean in this case? is it a log-shipping secondary, a mirroring failover partner or something else maybe? That will help...

I think the issue is that the port number at the end of your routing URL (5022) looks to be the same as for your Endpoint URL. Can you try...

The first issue can arise if you allow connections to the database on the primary replica before you have successfully added it to the AG. It can be a bit...

That's a schema modification lock. Did you have any online index maintenance jobs running at the same time or anything like that?

This is a good read: https://www.red-gate.com/simple-talk/sql/database-administration/alwayson-availability-groups-what-not-to-do-when-adding-databases/

It's even from Redgate :D.

Taking a transaction log backup will truncate the transaction log (i.e. make it available for re-use) but it won't shrink it. 

If your log is growing too quickly, increase...

Tom, I'm so very sorry for your loss.

Do you have any experience with non-basic Availability Groups, or is this your first experience of any of them?

I'd like to go but I'd need accommodation etc. and I don't think my company will spring for it. Maybe next year, if it's back in London.