I'd start by looking at step 1 of the activity calculatermaage. That's what's calling the objOpen method. What is it passing in? Is the data correct? Is it using an array with an index that is out of bounds?
As Mike mentioned - this could be problem with blob. your blob may be corrected, Please verify by hardcoding pyid for some object which you are sure and opened properly earlier without any issue. Otherwise if you are opening a case from repeating grid through a link then try "Open Work Item" option rather than "Open work by Handle" by passing pyID.