Could you say "vad är det för en fel" and would the meaning change? I think this "vad är det för en" structure compares the object of speaking, case-in-point to the wider class of such things, suggesting this particular object is somehow different?
It is "ett fel", but yes you can say that, with a little different meaning. It is more like asking what kind of error it is. It could also be a rethorical question pointing out a strange or stupid error.