Why is Vad var er pappas namn? not a valid translation?
er should be accepted wherever din is, so if you find cases like this, you should report them using the Report a problem button, and we'll add it.
Is 'kallades' wrong? I thought att kallas and att heta were interchangeable?
can't I say "Vad heter din pappa "?
I'm a bit confused now, what is the tense of "hette" here ?
It's the past tense, so you can't use the present (heter) instead, that will change the meaning.
Does it mean his father has new name or his father passed away?
It could be either. If you add again in English and nu igen in Swedish, it could also mean you've just forgotten what the father's name was.