I got help by a programmer who found that for some reason order_total module was initiatet twice.
So
$order_total_modules->process();

had to be deactivated in checkout_confirmation.php.

I have no clue how or when this happened or where the other "$order_total_modules->process();" is to be found but am glad that it's working again. So this thread may be closed.

Thank you for the comments!