Found the following error in the logs, should be investigated:
2016/10/18 19:33:12 [error] 9867#0: *212539 FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught TypeError: Return value of WMDE\Fundraising\Frontend\PaymentContext\Domain\Model\BankData::getBic() must be of the type string, null returned in /usr/share/nginx/www/spenden.wikimedia.de/release-20161018132825/contexts/PaymentContext/src/Domain/Model/BankData.php:24 Stack trace: #0 /usr/share/nginx/www/spenden.wikimedia.de/release-20161018132825/src/Presentation/Presenters/IbanPresenter.php(27): WMDE\Fundraising\Frontend\PaymentContext\Domain\Model\BankData->getBic() #1 /usr/share/nginx/www/spenden.wikimedia.de/release-20161018132825/src/Presentation/Presenters/IbanPresenter.php(18): WMDE\Fundraising\Frontend\Presentation\Presenters\IbanPresenter->newSuccessResponse(Object(WMDE\Fundraising\Frontend\PaymentContext\Domain\Model\BankData)) #2 /usr/share/nginx/www/spenden.wikimedia.de/release-20161018132825/app/routes.php(282): WMDE\Fundraising\Frontend\Presentation\Presenters\IbanPresenter->present(Object(WMDE\Fundraising\Frontend\PaymentContext\ResponseModel\IbanRespo" while reading response header from upstream, client: 87.176.149.66, server: spenden.wikimedia.de, request: "GET /check-iban?iban=de55680510040005079512 HTTP/1.1", upstream: "fastcgi://unix:/var/run/php/php7.0-fpm.sock:", host: "spenden.wikimedia.de", referrer: "https://spenden.wikimedia.de/donation/new?piwik_campaign=01-ba-161014&piwik_kwd=org-01-161014-ctrl"