Database and SQL errors in Who's Online page.
 

Whenever I go to the Who's Online page, I get an error.

Invalid SQL:

SELECT IF(userid > 0, 1, 0) as isuser, COUNT(session.userid) as online_users
, user.market_username_color,
user.market_username_glow, market_username_subscript
FROM session as session

WHERE session.lastactivity > 1314829461

GROUP BY isuser;

MySQL Error : Unknown column 'user.market_username_color' in 'field list'
Error Number : 1054

Do you or did you have the Point Market System mod installed? It seems like it didn't install or uninstall correctly.

I have it installed, what can I do to fix it then?

Apparently your database doesn't have some of the columns that are added when that mod is installed, so maybe try reinstalling it. (If running install doesn't work you might have to uninstall it first, although you may lose data if you do that). You might get better guidance if you ask in the mod thread.