Some additional things to check if uploading a clean copy of the admin files (and of course fixing permissions) does not fix the problem:

Double check to make sure there are no spaces, &, #, or % in your renamed "admin" folder. These can and will cause problems.

Double check /admin/includes/configure.php especially the 'HTTP_SERVER' and 'HTTPS_SERVER' defines. Make sure there is a trailing slash and the server name is correct (for example: 'http://www.zen-cart.com/').

The "magic" in this file usually takes care of configuring the next two settings automatically for you, but you may also have to change 'DIR_WS_ADMIN' and 'DIR_WS_HTTPS_ADMIN' to be the name you changed your "admin" folder to (for example: '/Wea42gc1/').