I am using version 1.5.1 on my newest website: www.personalizedpetid.com

I just had a security certificate installed so I can begin accepting credit cards. I have the payment module properly configured (from what I can tell).

I wanted to test the shopping cart as a customer. I can browse through the categories and items just fine, but when I try to add an item to my shopping cart (as a new or not logged in customer), I receive this message:

"Whoops! Your session has expired.
If you were placing an order, please login and your shopping cart will be restored. You may then go back to the checkout and complete your final purchases.
If you had completed an order and wish to review it, please go to your My Account page to view your order."

So naturally, I tried to log in as an existing customer and this is the message I received when I tried to add an item to my shopping cart:
"Whoops! Sorry, but you are not allowed to perform the action requested.
You are still logged in to your account and may continue shopping. Please choose a destination from a menu."

Something is terribly wrong here...the site was working fine with just PayPal as the method of payment. Customers could add items to their cart without any issue. Since adding the SSL and the authorizenet payment methods, a customer can't even add an item to their cart...let alone actually get to the point of checking out.

Please advise on how to fix this!!