Suddenly Paypal Transactions Vanish

Hi
Everything was working fine but suddenly, when a customer selects Paypal as payment, the order vanishes.

The payment still goes through paypal but a evidence of the order vanishes from the zen cart. Both from the admin side and from the customer side. :shocking:

Help please

If you're using Paypal Standard (IPN), switch to the much more stable Paypal Express and that issue should go away.

stevesh:

If you're using Paypal Standard (IPN), switch to the much more stable Paypal Express and that issue should go away.
Thanks Steve.
I am trying that. In debug it is asking for a NSS: CLient Certificate. What is this and where will I find it?

this is the error message in red at the top of zen cart step 2 of 3, payment checkout page:
An error occurred when we tried to contact the payment processor. Please try again, select an alternate payment method, or contact the store owner for assistance. () - (56) NSS: client certificate not found (nickname not specified

Found the problem.
Steves suggestion to upgrade to paypal express has worked.
The other problem relating to the NSS: CLient..... is to do with paypal address for sandbox (test) setting. I started looking at changinf the code in ...includes/modules/payment but have run out of time so set the module to live and no more problem.