When I used Zencart 1.5.4 and installed Paypal express checkout(debug), there are IPN errors as soon as someone place order in my Ebay account.
The system sent me 18 IPN error emails for each transaction. I posted some of them below:


Code:
#1
IPN PROCESSING INITIATED. 
*** Originating IP: 173.0.81.1  notify.paypal.com
*** Browser/User Agent: PayPal IPN ( https://www.paypal.com/ipn )


#2
IPN ERROR :: Could not find stored session {} in DB; thus cannot validate or re-create session as a transaction awaiting PayPal Website Payments Standard confirmation initiated by this store. Might be an Express Checkout or eBay transaction or some other action that triggers PayPal IPN notifications.

#3
IPN ERROR :: No saved Website Payments Standard session data available. Must be an Express Checkout or Direct Pay transaction.
Could be a test notification, or the incoming IPN notification is not actually a bonafide PayPal transaction.
NOTE: It is likely that all the following log content is meaningless or irrelevant.

#4
Breakpoint: 0 - Initializing debugging.

#5
Breakpoint: Flag Status:
isECtransaction = 0
isDPtransaction = 0

...


#9
CURL response: VERIFIED

#10
IPN INFO (cl) - Confirmation/Validation response VERIFIED

#11
Breakpoint: 1 - Collected data from PayPal notification

#12
Breakpoint: 2 - Validated transaction components

#13
Breakpoint: 3 - Communication method verified

#14
IPN NOTICE :: Could not find matched txn_id record in DB. Therefore is new to us.

#15
Breakpoint: 4 - Details:  txn_type=unique    ordersID = 0  IPN_id=0

   Relevant data from POST:
     txn_type = unique
     parent_txn_id = None
     txn_id = xxx

#16
IPN ERROR :: Could not find stored session {} in DB; thus cannot validate or re-create session as a transaction awaiting PayPal Website Payments Standard confirmation initiated by this store. Might be an Express Checkout or eBay transaction or some other action that triggers PayPal IPN notifications.

#17
IPN ERROR :: No pending Website Payments Standard session data available.  Might be a duplicate transaction already entered via PDT.

#18
NOTICE: IPN Processing Aborted due to missing matching transaction data, as per earlier debug message. Perhaps this transaction was already entered via PDT? Thus there is no need to process this incoming IPN notification.
Could someone help to fix it? Thanks very much in advance!