Custom validation for text entry form

The custom validation is creating the problem, because it is failing if there is no answer. Since empty is not a number.
Add OR conditions which allows validation to pass if the fields are empty.
Alternatively, you can set default choices to "0", so that if your respondents don't touch a field, the validation passes.
Hopefully that should fix it.

Thanks so much for your reply!
I think I read about a similar problem in another thread, it doesn't really make sense to me though. I do understand your answer, but while testing this block, it was not the case that I tried to leave the fields empty. Why wouldn't the validation let me pass when I entered "50" and "50", for example?
I just added OR conditions for empty fields and now the question works as supposed to. So thank you for your solution :) I just don't understand why there was a problem at all...

I'm glad my suggestion helped, but 50 and 50 should have worked without it. Adding the OR conditions should have no effect on it. Just check everything once again to ensure you aren't overlooking any validation.

Thanks for replying again.
I just played around some more: When I delete my validation INSIDE the form question itself (as stated above, I set the forms to be "short" and take a "numerical value"), 50 and 50 works.
This, again, makes no sense to me. But well, at least it works...