by Ken Task.
Looks to be a server operating system web service 500 error ... a generic error which could have many causes.
Check web servers error logs for clues.
If remotely hosted on windows, there is a config item to supposedly help with hosting software that might use /admin See the config-dist.php file in your Moodle code.
Also, in that same file ... config-dist.php ... Section 7 ... tells you how to add debug lines to config.php so moodle debug reporting will be turned on and might be a little more useful than 500 errors.
'spirit of sharing', Ken