Customer log in error

I am using Zen Cart v1.3.8a/v1.3.8 and cold steel template.

I am new to zen cart , and some of my customers had complaints they are unable to log in and check out to purchase items, however i am unable recreate the error on my end using internet explorer and firefox

I have tried the tips onhttps://www.zen-cart.com/tutorials/index.php?article=312tips

Could anyone help me recreate the error so I can solve it? the shop is at http://drsecretstore.com/shop

thank you !

Some of the complaints i received:

1. Unable to log in

2. Unable to check out, error when checking out

3. Postal code not accepted (International Address like Vietnam)

4. Page goes back to first page when I check out

i am using the current 1.3.8a version.
http://blendedelegancellc.com/shop/index.php here is the site

I am also having these problems. after a customer clicks the login link it goes to a blank page this link http://blendedelegancellc.com/shop/index.php?main_page=login&action=process i would like this to go to http://blendedelegancellc.com/shop/index.php?main_page= this is the main page. this also happens with the creat account page after i click the submit button, however the info is stored in the database. i hope i have made some sense of what is going on.
HELP PLEASE:(:(:(