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In my rush to work out what was wrong, the below was in fact completely incorrect. What happened was that some code got rolled out that expected a change to the DB table, but the change hadn't been applied to the DB. Code then trying to pull data from the DB table was failing because the column wasn't present, making lots of data look "NULL" internally.
With the appropriate database update applied, everything should now be working correctly, and no data was lost.
In the web interface, users can set a "default personality" that's used when they go to the Compose screen. Due to a database query error, that value has accidentally been cleared for all users, so currently the web interface will be picking the "first" personality for each user by default.
We're working on retrieving data from a backup database to restore the values for all users, I'll update this ticket when we've done that.
If you need this fixed immediately, you can manually fix this by going to Options -> Personalities, selecting the personality in the list at the left, and clicking the "Make default personality" button to make it the default. When we do the database restore, we'll leave any values manually fixed so we don't override them.