what a cute and beautiful error....but its very simple 
my issues was also the same and I resolved it ..
Once you put WSDL URL and click on next you will get webservice name on the next page ...
and when you click on next on the mail request and response message page.. you will get this error on top.
In my scenario .. I was testing a web service which is via SAP PI.
1. Chk the error message.
2. Go to -> Runtime Workbench -> Message Monitoring -> chk for this message -> Go to Audit Logs.
3. you will get the final error here with the reason that why is that your web service is not working fine. 
4. With the help of ABAPer check for those particular fields in SPROXY, and try to test the same again .. 1000% it will work .
Thanks
Zeeshan