Having poked around further it looks like a home brew system for example the class throwing the (spurious) error when you use service = wfs instead of WFS is si.bron.GISRepSystem.WFSExportServlet.WFSRequest.WFSRequestImpl isGetCapabilitiesGET
Combining the inability to handle case in the request with the completely wrong SRS listed in the capabilities file it looks like a badly implemented home brew WFS which would explain why you can't connect to it.
I'd suggest contacting the owner but they skipped that part of the capabilities response.