You might ask your webhost to help you review your server's errorlogs to determine what is getting recorded when that appears.
If you enabled SSL during installation, you might try turning that off as a test. Also test whether the account-login or create-account screens work properly in the storefront.
https://www.zen-cart.com/tutorials/index.php?article=15
Also, try renaming the .htaccess files under your admin folder to .htaccess_OFF in case they are attempting to do some sort of rewrite that your server doesn't like. You may even need to do that in non-admin sections, esp if you're running on a Shared-SSL configuration or have Zen Cart in a subfolder and have various rewrite instructions in your .htaccess elsewhere on your site.
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