"Unable to save request" Errors

Every page load (including /registry-pe when not logged in) generates one or more entries in error.log like

2024-02-27 00:58:13 warning: Unable to save request. This is probably due to concurrent requests.2024-02-27 00:58:13 warning: SQLSTATE[HY000]: General error: 8 attempt to write a readonly database

These don't seem to affect anything, but we should track down what's causing them and fix it.