Zc v157c
Zca bootstrap 4 v3.1.2
PHP 7.4 or 8.05

Hi guys, I am uprading to the latest Zencart version 1.5.7 with ZCA 3.1.2 and I have a problem at checkout where the payment selection methods page comes up blank(/index.php?main_page=checkout_payment).
The error in the log is
PHP Fatal error: Cannot declare class ot_coupon, because the name is already in use in /includes/modules/order_total/ot_coupon.php on line 14
I tried solutions from older threads without succes. The ot_coupon.php is the only core-file overwrite of the template which was copied replacing the original(no changes on original distribution). Any tips are appreciated as the next stage is to rebuilt the installation from the begining.