Hi,
I had a similarish issue with the 7.1 upgrade on my dev site but in admin.
I have used the automatic updater for all versions since v7, I had also uploaded the v7.1 beta.
I'm running php v7.
Not sure where in the process this might have occurred but one of the files /modules/gateways/internetsecure.php was encoded by the ionCube Encoder for PHP 5.3 and cannot run under PHP 7.0
which causes the requested page to return just a white screen.
I downloaded the v7.1 release manually and checked that file is now not included so removed it and now the admin payment gateway pages now works.
Regards
Keith