"Internal Server Error" when I try to add a new customer

Hi,
I am new to zencart and have just started to test the ordering process by creating fake customer names. When I press the "submit" button I get this error message:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [email protected] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

However, when I look in admin --> customers, I see that the customer info I entered actually has been registered.

But a couples of times, doing nothing differently than before, I have gotten this message:

Email Error: Could not instantiate mail function. Please check Admin->Configuration->Email Options->Email Transport."

I am not sure why this message has come up only sometimes.

Anyone know what's going on? I looked through previous posts and found one with this same question, but no one had replied to the post.

My shop is here:
http://rigelstuhmillerportraits.com/shop

Thank you very much!
--Rigel

Oh the fun "500 internal server error" message.
You'll have to check your hosting account for access to the server's errorlogs ie: the apache log files which explain the cause of Internal Server Errors and other errors it encounters.
You might have to ask your hosting company for help.