Sorry, I didn't have a chance to post back here this morning. How were you adding users to the system? Was it just by having them join through the webapp?
The error message you got usually indicates that there was a null value in a SQL column that shouldn't usually be null. From both of those errors, it seems like it might have been the Nickname column of the Users table. If you have access to your SQL database, maybe try seeing if SELECT * FROM Users WHERE Nickname IS NULL returns anything. If that one doesn't work, I'd check all of the columns in the Users table.