This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
A close look at the generated could help. Let me guess: When parametrising the XML element names, one of the names you choose was "Parametry symulacji"? As white spaces are not allowed within XML identifiers the parser treats symulacji as a second identifier, which must be an attribute name at that position, but is not followed by a value.