Thanks for sending me your FTP login. Removing my plugin was a good test to determine if that was what broke your site. With my plugin removed we can see tat the error is still there and is not related to your error.
At first I thought that your server was just having trouble reading or writing to session files but then I noticed that the session file 'sess_3fa452ae65160f235f6c2a14d3c5603f' in your originally reported error is the same as the file name in the error that I get when I view the page. This indicates to me that it is not actually generated by the server trying to access the current session (because the server would generate a different file name for each visitor). Instead I believe that this file was was intended to contain malicious code but was either deleted or never written. Somewhere on your server there is a file that tells the site to include this file, and because the file is missing it is causing this error.
I have downloaded and search the contents of all the files on your site that I have access to with the FTP credentials that you gave me and there is no mention of this session file. If this file is included from outside the public directory of this site then I would guess it is either in a php.ini file or .htaccess file in a directory higher up then this site directory.
If you want extend my access to your server to include the parent directories then I will continue my search. However, maybe I have provided enough clues that you or Jason can find the include directive wherever it's hiding.
Please keep me posted and let me know if I can be of any further assistance.