After the 2010 upgrade, we have come across a strange issue with regard to the column ordering related to the Issue content type. We have a list based on the default Issues list. When we try to change the order of the fields on the list form by clicking on the “Column Order” page under Content Type settings, we are being prompted by an Server/Application error which says “Object Reference not set to an instance of an object”. The stack trace refers to the nullreferenceexception related to the ChangeFieldOrder.aspx application page.
Export the list and Import it back. The issue is gone.