Here's my answer. As the Screen was working fine with oct version and it is showing error in feb version, I would suggest that you regenerate your CASE and ACTIVITY table simply by going to Customize > Tables/Fields > then click on the Cases table first and click Finish Editing Fields. Again same process with Activity Table, click on Activity table followed by Finish Editing Fields. This process should regenerate the tables and should solve the problem. I have tried it and it is working fine on my end.
If you are still having trouble with this, please let me know.