Problem with my opencart edit orders not working

The problem I am experiencing:

When I try to edit an order from opencart admin by adding new products or trying to delete ones already in the order I get an error message. This feature of editing orders is extremely important and I need to get it fixed somehow.

Note: the problem existed before I installed an extension (+VQMod]Admin One Page Order View and Edit) that changed the look of the admin order view

I have an extremely basic knowledge about opencart which basically consists of uploading files via FTP. I tried googling the problem, and could not find a solution apart from the fact that opencart may need to be reinstalled. However, I did not want to try to reinstall opencart myself as I have added various extensions to my site, as well as edits of other files, such as duplicated payment methods, and I didn't want to loose any of these changes.

Also whilst the system admin language is in English and I try to open extentions > shipping I get only a blank page. I can only access it when the admin language is Russian. This is fine, but I presume this problem can easily be fixed whilst fixing my edit order problem?

I need someone who is able to fix this edit order problem by reinstalling minimum files so that no other parts of my site are affected.