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According to section 8.4.1.1.3 of the conditions of carriage skateboards are "blunt instruments" and are not allowed to be carried on board. I assume this extends to longboards, too.
On Twitter Eurowings confirmed that skateboards can't be carried on board. They refer to the sport equipment page which does not mention skateboards/longboards. Other sport equipment is priced at 50 € according to the page. Likely you could also check it as regular luggage.
I have no experience though and in practice it might look different.