My web host upgraded the database security settings, and must have used some sort of automated system to automagically parse through my sites to identify configuration files with database access information. Where it detected these, it updated them with the new settings.

Except for Aiera, it seems. The presence of the forum evidently caused some confusion; the forum’s configuration got the update, but Aiera’s was somehow missed.

Not that it wasn’t an easy thing to fix, mind you.