Check to see if it is trying to use SSL.
If, on the initial setup, you chose to have the admin area secured and
it was either configured it incorrectly or was set to use the shared
SSL, it will fail. The shared SSL doesn’t work in the admin area (a bug
in ZenCart). You can correct the setting (if you have your own
certificate) or just turn off SSL in the configure.php file found on
their site under \Zencart\admin\includes. To turn SSL off, change the
following line in the configure.php file: