Correction of Wordpress theme error

The site will work without the www. prefix "http://vesavaltonen.com" is working fine. I am using a theme which does no longer have support by its developer. Everything works fine with the latest version of Wordpress until I will post any text to be published. Then entering "wp-admin" it will produce the following error: Cannot modify header information - headers already sent by (output started at /home/xxx/public_html/wp-content/themes/mcmac-10/[login to view URL]) in /home/xxx/public_html/wp-includes/[login to view URL] on line 934 Also because of the above it will not work with the www. prefix ([login to view URL] will not work). Correction of the above errors. Might be very simple (an extra white space in the code) or some error in the code structure.