Server 503 Error with Admin Login or any customer login

[split63]

V1.5.7c
php 5.6

We have made no changes. This morning a customer contacted us to say he is getting an error upon checkout. We tried to admin login:

"503 Service Unavailable error was encountered while trying to use an ErrorDocument to handle the request."

If I try to login as a customer, same error. But, if I click back, I find I'm actually logged in as a customer....so it worked, but gave the error. I can then see previous test orders in the account. While logged in as customer, if I click "Shopping Cart" or "Checkout", I get the error.

Any ideas?

HTML is working, PHP is working, general product database access is working.

Its a godaddy hosted site, and I have been on hold with them for hours...