Where is sorting customization for header bar arrows in list view located?

We are trying to move to a new  virtualized server. 

I have changed no code and only the MySQL version has change from 5.5 to 5.1

Now the sorting arrows in the list view header in one of the custom modules doesn't work. 

I am trying to find where any customization to this module has in the way of sorting.

Note: The advance search still sort correctly.