Hi Bob,
In general this type of error comes, when you are trying to set the value for XML DOC Column (In your case it is BAPI message) in a Row Action block but that value is not available .i.e. If BLS action block is not able to found the reference of the assigned input.
Can you check if BAPI output message assigned to action block is valid or has not changed after upgrade ?
Example - If I assign a local variable to Row Action block whcih is msising or not defined then it throws this kind of error.
Thanks