Connection refused — log says it's a bug but it's working OK elsewhere on the same server.

I'm not sure how to report or seek help with this issue. I'll be happy to post all the details here if that's appropriate, otherwise I'm really just looking for a pointer towards some help!

I run a server with a few WordPress sites. All of them are working except for one: requests for that site lead to signal 11 and a refused connection, and the log says I should report it as a bug. But the first step is to check if it's a known issue, and I have no idea what in the error log is significant and what isn't.

I can log in to the same database on the command line without any problems, so it's not the credentials.

I recently upgraded the server OS and MariaDB, and I was experimenting with memcached to see if I could improve performance, but the site was definitely working for some time after I did all that, a couple of days at least.

How do I go about reporting this issue? It seems hard to know how to approach it because it almost looks as though it might be a WordPress issue, except that the other sites are working and it is MariaDB that refuses the connection and tells me there might be a bug.

I would really appreciate any advice!