"Register" error

Every time I click on the "Register" button, and I fill in the details requested and click on "Register" to have my new username registered, I get the following message:

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [Ed: email with-held], and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

Any ideas what's going on, and how to fix it? Thanks!