Hi @asmecher YES plugins/paymethod/paypal/vendor/symfony/event-dispatcher/EventSubscriberInterface.php exist and now the problem of blank page seems to have solved. but on that screen there is a message and payment is still not working : A transaction error occurred. Please contact the journal manager for details.
EDIT: LIVE API CREDENTIALS are working. SANDBOX API CREDENTIALS are not working.
EDIT: The issue of transaction error seems to have solved by using LIVE API CREDENTIALS in Paypal configuration.
Best Regards