The error code means there was some problem with processing the request on the side of the server.
Theoretically it can be anything, but assuming you are trying to use a well-functioning web service, most chances are that you have sent wrong data that the server cannot process.
It can be due to wrong format (e.g. because of a problem in the WSDL import), or wrong data (e.g. illegal values for fields).
One way to check thing is to look what exactly is sent to the server and compare it with a sample you know works OK (if the service provider offers such samples).
For best results, use the Firefox browser..