Comments - Connection refused — log says it's a bug but it's working OK elsewhere on the same server.
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Signal 11 means the mariadb server process crashed, and that is usually a bug in MariaDB. The refused connection is caused by the crash, connections will be refused after the crash untll the MariaDB server has completed restarting again.
The reason could be that the wordpress size that causes the problem is using a specific query or usage pattern that triggers the bug in MariaDB. One such problem that was already reported is this one:
https://jira.mariadb.org/browse/MDEV-33533
You can check if this looks similar to your problem. Otherwise, you can use that bug for inspiration about how to file a bug about your issue.
Thank you so much for your help. Sadly, I'm trying to fit fixing this in around work, so if I seem unresponsive it's that and not ingratitude!
I'll let you know how things go when I get the time to follow up. Thanks once again.