Error log generated with message: Invalid IPN

Hi,

Since 2-3 month I have a big issue with Payplans, when a user pay with Paypal sometime about 1 for every 3 times I get an Error log generated with message: Invalid IPN. I have just updated Payplans to the latest version but it did not fix the issue.
Error log is enclosed to this message.

My IPN url is perfectly set in Paypal since many years and I don't think the issue come this.

My IPN url :
http://www.rikoooo.com/index.php?option=com_payplans&;view=payment&task=notify
Set on : Receive IPN messages (Enabled)

You can login to my back-end as administrator and have a look.

Thank you for your help

Erik