The server actually ran fine again for almost a year, to now have just stopped three times within the last 30 days.
What the three recent failures had in common was that each time the last logged message was the first line of a two-line failed SSH login attempt message.
I have now taken a few steps to harden the SSH setup, and will lock it down even further as soon I’m back home next week to only accept requests from my fixed home IP address.
Now anxiously waiting for the next failure to happen — or better not happen — to see whether this really fixed things, or whether there’s actually a different problem and the SSH log messages only having been a coincident …