We recently attempted an upgrade on https://amishoutfitters.com/catalog/ from Zen Cart version 1.3.9h to 1.5.6b, and when attempting to access the admin page, I get a blank screen. I did some basic troubleshooting before rolling back the site to a backup of the 1.3.9h site and still got the blank admin page. There are no Zen Cart errors generated when accessing the page, and our hosting provider has said that there are no Apache errors either.

I did the upgrade using the instructions included with the download. We are currently using PHP 7.0 (I switched to 5.3 temporarily when rolling back the update). I've tried multiple times to upload clean files to the site, and I've checked multiple times to make sure the paths in the configure.php file are correct. There are no customization to the site beyond text changes on the customer side. The issue seems to be isolated to the admin page, as the rest of the site seems to be functioning normally.

Without any errors I'm at a loss trying to figure out where to start troubleshooting the issue and was hoping for some assistance.