I do not receive my orders, but payments show up in Paypal. No trace of transaction in the IPN menu in the back end either

My cart sells only virtual products with free shipping, so the shipping page is skipped (all products have free shipping)
freeshipper module is enabled
my paypal address is the primary, I tested with the IPN check script, all is OK according to the script:

Code:
IPNCHECK.PHP - Version 1.0
IPN TESTING - POSTING to PayPal via: ssl://www.paypal.com:443
IPN TESTING - Confirmation/Validation response: RESPONSE RECEIVED - Communications OKAY
Script finished.
My Paypal settings:
the IPN is activated
the return url is provided (the one that zencart provides in the module)
payment data transfer is on

I turned on the debug, placed a dummy order and found out that :

between breakpoint 2 and breakpoint 3 the following message: IPN WARNING::Could not find matched txn_id record in DB

Breakpoint 5: txn_type=unknown

and the last email starts with:
IPN WARNING:: Could not establish txn type unknown

Thanks for your help on this matter.