Server 503 Error with Admin Login or any customer login

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...

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Here is a screen shot. I read that 503 is a server side error.

Attachment 20203

Okay, are you ready for this..
The Godaddy environment changed in some way they could not say.... but suddenly the Zen Cart site was reacting to the .htaccess file contents.
There was one line that jumped out at me as unknown, so we removed it and the Zen-cart site started working again

The line itself looked malicious to me, but I checked an older version, and it was there too.
This is the entry:
AddHandler application/x-httpd-alt-php56___lsphp .php

Gotta (not) love GoDaddy! Thanks for the update with the solution to your issue.

More on the GoneDaddy resolution at https://www.zen-cart.com/showthread....Customer-Login