I finally got myself a working external service.
But when I want to test it (in <server>:<port>/wsnavigator) and I fill in my 2 import parameters it returns nothing.
I went to check for the problem and one of the import parameters is not being passed on to the RFC. It remains empty.
After executing the service you are able to see the SOAP messages that were sent and returned.
When I look in the SOAP message that was sent (with my 2 import parameters) I see the value "AA==" for the import parameter that is not being filled.
Does anybody have an idea why this import paramter is not being filled in?