I found this error in the log files after the customer had trouble with payment:
--> PHP Fatal error: Uncaught TypeError: Square\ApiHelper::serialize(): Return value must be of type string, bool returned in /home2/rcracean/public_html/silversalt/includes/modules/payment/square_webPay/square/square/src/ApiHelper.php:113

I did not receive any Square Alert mail about this error.
I use Zen Cart v1.5.7d and php 8.0.25

Has anybody any idea what could cause this error?