I would start with making sure Event Espresso and the add-ons are up to date.
There is also a simplified Transactions page template that can help with problems with incomplete payments that you can add to your WordPress theme. There is more information here:
The support post ‘Automated Email after Success gives link to Payment Details – Payment incomplete’ is closed to new replies.
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Support forum for Event Espresso 3 and Event Espresso 4.