If you are receiving this error, then the Netbiscuits platform encountered a server error while requesting your BiscuitML page from your backend.
The problem is most likely your backend - please check the server logs on your machine.
Also, try accessing your backend directly and not via the Netbiscuits platform, and make sure it delivers the correct markup.
It's recommended to enter the action without the leading "./" - have it start with "servlet/"
Lastly, you should make sure that your content type is set correctly (see my earlier post)
On an unrelated note - the notation for BiscuitML has changed - the one you are using is now deprecated.
The better way would be to use <FORM action="something" method="GET"> ... </FORM>
The online
BiscuitML Reference will give you more information