OK sorry for the downtime. What happened is that I added a feature which added some new settings.
This is all fine and good.
Each user's settings are stored as pickled python objects in the DB. Not the best choice but also not really a problem. Weirdly when the updated settings were written back to the DB the data was corrupted by mysql!
"Invalid utf8mb4 character string: '800495'"
When the settings would get read back out moa would crash.