Server Error (500)

[ToeC]

I've successfully added two domains but attempting to add another results in:
Server Error (500)
That's the only content in the page source. I'm attempting to create a Master / Private but I also tried to make a Slave with the same result. No further info is available as far as I can see.
The nameservers have already been changed over to ydns with my domain provider.
Thanks for any help.