Well, I've fixed the interest problem.
The problem was, it was failing to look up the user interest information (namely the extra interest data) when it updated the account information when collecting interest.
This also had the side effect of making it possible to 'cheat' the interest system and obtain interest more than once a day by calling the interest script directly.
These problems have now been fixed.
However, if you have bought an extra interest item or any other item for that matter YOU MUST USE IT from your inventory screen for it to take effect.
Also, on a side note, I've made it so you can't lose more than 200 ore on a casino item.
Well, this issue has been resolved, so I'm locking the topic.