Quote Originally Posted by relish View Post
Running it on localhost XAMP
Please provide any Zen Cart error logs corresponding to the message. These can be found in the log file directory you configured when installing Zen Cart. Log files generated when browsing the admin pages usually have "adm" in the name. These may or may not have been created.

The XAMPP Apache Error logs are usually at something like "\xampp\apache\logs\error.log". Please look through your error logs and find the line corresponding to the 50X error message. These are always created when Apache displays a 50x error unless you have disabled the error_log for apache. If you have disabled the error_log, please turn it back on.

Looking at the contents of these logs should give a better idea of the underlying cause.