i had this problem and tech support gave me this answer and it worked

The issue you have experienced is a known one with the admin panel. Please note that the administrative panel is easily affected by rewrite engines included in .htaccess files in parent directories.

I have just placed a .htaccess file under your /cart/ directory with the following text in it:

RewriteEngine off

This prevent the effect of all rewrite engines from the main directory to affect your Zencart application. As far as I can see I have been able to login with the provided username and password: admin/admin.