They must have third party customization or ISV code that has not been upgraded and recompiled against GP 2016.
Please check the Dynamics.set and the version numbers of all dictionaries.
If you have GP Power Tools, the Product Version Validation window will show all the dictionary version numbers.
To get this error they must have some code that was compiled before an optional parameter was added to the IV_Check_Location global procedure in March 2012.
If they had a trigger against this script it would give an error on startup when the trigger was registered. However, if the custom code is calling the script it would fail at the time the call is made.
The code does not need changing, it just needs to be recompiled against a GP v16.0 dictionary.
Did they recently update to GP 2016?
This is the risk of using dictionaries that have not been updated for the same version of GP and Dexterity.
Regards
David