It isn't possible to define a namespace without a prefix.
namespacePrefixes: {
'http://ris.bka.gv.at/Search/1.3/OGD': '',
'http://www.w3.org/2001/XMLSchema-instance': 'xsi'
}
It adds
xmlns:xmlns="http://ris.bka.gv.at/Search/1.3/OGD"
but it should be only
xmlns="http://ris.bka.gv.at/Search/1.3/OGD"
ximex/ris-bka#4