The database server started refusing many/most new connections with an error related to free lock memory.
This appears to be a recurrence of a problem first encountered when we massively increased the background worker queue.
I quadrupled the setting that controls lock-related memory and restarted the database software to resolve this problem for now.
I have ticketed an issue to try to find a way to monitor free lock memory to try to get advance warning of this problem in the future.