Your problem seems to be investigated on the GeoServer developer mailing list in this thread
Summary
Cascading WMS from the Polish geoportal is failing because the server does not accept Java as a User-Agent in the http headers.
I recommend to contact the administrators of the geoportal because this behavior it preventing the use of the WMS service with all Java based clients which are configured to send the jre version as User-Agent. IMHO it is a normal and right thing to do. It is hard to believe that blocking clients that way by http User-Agent headers from a national INSPIRE view service is intentional.