Update: this seems to have been caused by a server upgrade of McAfee ViruScan Enterprise... moving from v8.0i to v8.5i
When we downgraded back to v8.0, Moodle (i.e. MySQL database tables) responded normally again.
Yikes! Another user on a MySQL forum solved it by excluding all the MySQL databases from the virus scan. That sounds like a kludge, no?
Does anyone else have experience with this? We will bother McAfee customer service about it, to see what they say.