Manage categories in Magento broken, Magento Error Fix

Dear applicants, at first please have a look at the screenshot which is attached to this project post. This shot shows an error which occurs when we want to "manage" our categories. What happens: After we click on "manage categories" in the admin menu the page loads and we it's finish it shows this "orange error box". What I need: I need a fix as soon as possible. What we want: I only want you to fix the error WITHOUT reinstalling or deleting magento. It's very important that NO data gets lost during the fix! We provide you with everything you need. This job should be aroun 15 USD nothing more ... and NO in front payment. We only pay when the job is done, because of very very bad experinces we did on Freelancer.com Note: you have to send me a documented , how and where did you fix the error. Please send us a PM if you need further information! regards