Hello fasterthanlight,
If you receive the message 'EPIC FAIL' while using the webduino library it would indicate to me that the url you are using is not understood by the Arduino server.
I have not fully taken in your code yet, but where this message can originate from is where you add server commands, such as the following,
webserver.addCommand("allesOn", &allOnCmd)
The first parameter "allesON' is the URL tail section and the second parameter is your function to handle the command.
If you either do not have the specific command added to the server list as above or you do not have the function, then you will get the "EPIC FAIL" message.
Hope this helps somewhat.
Paul