A couple of weeks ago, one of my clients alerted me to the error happening on their website - www.doctorjay.net. At first, I noticed a bunch of error code lines at the top of the homepage, all referring back to various php files. When I logged into the admin section, I immediately saw "--Add Me" error all over the login screen (ex. --Add Me: login, --Add Me: password, --Add Me: submit, etc., etc.). They were also found all over the admin menu after logging in, most noticeably in the drop-down menu. Two weeks ago, the CMS version was 1.2 and given that it had been working fine on it for over a year, my first suspicion was a corrupted database somehow. The server is Linux with Apache and PHP through Earthlink.
Before reverting to a backup of the database, though, I wondered if updating the CMS would help fix some problems. I've upgraded to 1.6.6 and got similar errors. I've deleted all the CMS files from the server and started from scratch, uploading the 1.6.6 CMS files and only keeping the config.php the same as before. Now, my homepage index.php gives the "site down for maintenance" message, and my admin homepage only displays the drop-down menu but nothing below it, so I can't access templates, global settings, or anything else.
Here's a screenshot of the current drop-down menu with the "--Add Me" error:

Has anyone encountered this problem or have leads of how to solve it? Any technical specs I'm forgetting about that would in giving feedback? Thank you so much for your help!!
Rob