When saving an order with inline credit card payment, this error occurs:
PHP Fatal error: Can't inherit abstract function Netresearch_OPS_Helper_Payment_DirectLink_ RequestInterface::handleAdminPayment() (previously declared abstract in Netresearch_OPS_Helper_Payment_DirectLink_Request) in /magento/app/code/community/Netresearch/OPS/Helper/Payment/DirectLink/Request.php on line 14, referer: http://[shopurl]/checkout/onepage/
This problem can occur when a very old PHP version (older that 5.3.9) is used. It is a bug in PHP, see https://bugs.php.net/bug.php?id=43200.
PHP needs to be updated to resolve this.
Note that PHP 5.3 has reached its End of Life (see http://news.php.net/php.announce/99), therefore it is recommended to use PHP 5.4 or higher.