webcentric As suggested here, You may inform your hosting provider to provide the server error log as currently, there is just blank white screen with no error to be shown after switching on to PHP 7x which could have related to some extension/code causing problem henceforth enabling us to trace the root cause
Also, Does the blank white screen still appear if you switch on to PHP 7x with default template assigned ?
(The earlier error had been coming from configuration.php fle)