Hi
I had the same problem.
The reason was a modification in checkout_confirmation.php for an older postfinance module.
Search in checkout_confirmation.php:
if (is_array($payment_modules->modules) && ($$payment->code == 'postfinance')) {echo $payment_modules->process_button();}
Delete or comment out the line like this:
//if (is_array($payment_modules->modules) && ($$payment->code == 'postfinance')) {echo $payment_modules->process_button();}
I hope it helps
Greetings
Rolf