Why is Multisite "Add New" functionality not working any longer?

I have added one subdomain site, but now the “add new” site button creates a site in WP, but when trying to access the site or dashboard I receive an error:

Internal Server Error

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

Please contact the server administrator and inform them of the time the error occurred, and the actions you performed just before this error.

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

Apache Server at folsom.pathwaysforveterans.org Port 80

primary site:

http://www.pathwaysforveterans.org

I need to create several additional sites ASAP, but am stuck. Please help!