svc test webpage partially garbled

Sometimes a webservice will show a partially garbled website when you enter the address of the .svc file in the browser.

I see some of the text as for instance:
"
<BR/>
You have created a service.<P class='intro'>To test this service, you will need to create a client and use it to call the service. You can do this using the svcutil.exe tool from the command line with the following syntax:</P> <BR/>

"

and when that happens the webservice will time out when called.

What can cause this? How do I fix it?

I have checked, that the IIS is running, the app pool is running (.net 4.0)

Thanks in advance.