Joomla Developer - To Solve Category Problem

Our company website is a Joomla site which we have developed in-house, with many modules including Jomsocial, Myblog, DTRegister and many more. Recently we have discovered we get an error in the back end when we try to add a new category and can't create any, we need some help debugging the problem.

This is holding us back on adding new sections to the site. The site is our second site, which we copied user data over from the orignal one.

We are based in Stoke on Trent, Staffs.

More Info on the error, when we click 'add new category' we get the following message at the top of the screen - Table class JTableCategory not found in file. And if you then try to save a new category - Fatal error: Call to a member function bind() on a non-object in /home/dev2/public_html/administrator/components/com_categories/admin.categories.php on line 431.

Joomla Version 1.5.15