You are getting the below error message when you click the "Export To Excel" Action, to export a Category List to an Excel spread sheet from within the Administration Component.
The problem here is that the sequence of the categories somehow has become corrupted so it doesn't know in which order the Categories should export and it will therefore error.
The solution to this is very simple though, all you need to do is create a new top level Category. When you do this the Database will autoamtically resequence all the Categories and therefore fix this problem.
After you have created the new Category you can without any problem delete the newly created Category again.
If you are getting this error message when adding a new Category, please see here.