Can you decrase the "min memory per query" even further to 512KB to see whether same issues occur less frequent than before?

When same problem happens again, Try to removes all cached queries from the procedure cache by "DBCC FREEPROCCACHE" without recycle SQL Server to see if you can run any queries.

Did both SQL Server instances (active/active) run in same node when you experienced the problem? Are any other applications competing memory resources in same machine? Is "min server memory" be set to default 0 on both instances?

You may also start to trace which queries runs prior such error messages appear.