can't see completed and payed orders

I have two online stores, one using v138a version and the other 1.3.9e
Both have been working fine for many years.

I'm having problems lately with the first version where customers order items, pay for them with paypal, I get a message from Paypal saying I got paid, however no order is shown on the store, the stock quantity does not change, the customer history does not show any order has been made.

I've tested with a dummy user, confirmed it is not working.
It does not happen with my other store using ver 1.3.9e

When I set IPN debugging I get towards the end of the log file /includes/modules/payment/paypal/logs:

IPN WARNING :: Transaction was not marked as VERIFIED
IPN FATAL ERROR :: Transaction did not validate

on 1.3.8a version I have: PHP Version: 5.2.17 (Zend: 2.2.0)
on 1.3.9e version (where it works fine) I have: PHP Version: 5.6.40

Any idea what might be the problem?
Thanks,
Amnon

FWIW, I'm quite surprised that the sites are still working:

1.3.8a ..... released in December of 2007
1.3.9e ..... released in June of 2010
PHP 5.2 ... released in January of 2011 (and I'm especially surprised that your webhost is still supporting this version)

Barring the above, while you don't identify which of the PayPal methods is currently in use, PayPal has become more restrictive over the past 16 (!) years so that could be the problem with your 1.3.8a installation.

You really, really, really need to upgrade both the Zen Cart code (currently at 1.5.8a) as well as the version of PHP (most recent released version 8.2). Many vulnerabilities have been corrected in both over the past decade.

Thank you.
I will look into it.