Hi Dr Goebel and welcome,
It sure is frustrating that an update would cause a break. I'm not sure why this broke but I have a simple solution.
Replace the code with declarative logic in the form. You can do your calculation just fine without code.
Just add a rule and an action to set the value with the calculation. All Xpath and it will work fine. You will need to learn about how to create a declarative xpath function in InfoPath, but not a big deal. In addition to actually working, the other benefit will be you don't need code (so less maintenance hassle).
:)