Webservice works only after local request

Hi All,
I have a strange Web Service issue.

The functionality is kept very simple. Send a text and return the uppercase value of it.
The webservice in installed on a separate server (ServerWS). Created a small C# program on my local machine that sends text and shows the result. I can get that working but…….

After the service is started I have to go to the ServerWS, open the browser and execute the URL (e.g. http://localhost:7047/DynamicsNAV/WS/CompanyName/Codeunit/MakeUppercase)
It is only after this action that requests from other machines are successful. I can change the functionality (e.g. add an extra A to the result) and it is effective immediately.

The same happens after an error occurs. The service is still running but every next request results in an error (in the Browser error 500). Until…. yes, I execute the URL in the browser on ServerWS.

Nav version is 2009 SP1.

Any ideas?