I had the same problem and went crazy several days. No I solved. Iread all the threads on all the forum possible. Here my lesson learned.
Zencart 1.3.8a from the scratch
some addons but the did not created problems
once you confugured all PayPal profile correctly and the PayPal Zencart IPN modole too
Once you have test your communicatios with paypal servers with ipncheck and works correctly
once you are aware that your host company in not godaddy
take a look tou your PHP logs
if you find something similar
1292 Incorrect datetime value: '02:20:07 15 2009' for column 'payment_date'
at row 1
in:
[INSERT INTO sap_paypal (order_id, txn_type, module_name, module_mode, reason_code, payment_type, payment_status, pending_reason, invoice, first_name, last_name, payer_business_name, address_name, address_street, address_city, address_state, address_zip, address_country, address_status, payer_email, payer_id, payer_status, payment_date, business, receiver_email, receiver_id, txn_id, etc...
you catched the problem.
i founded it before http://www.zen-cart.com/forum/showthread.php?p=484125
but did not linked this solution to my problem.
you have to turn off "satic mode" in MySql
run sql query
SET @@global.sql_mode= '';