18710 Session Management Issue

I have a script with an admin login. The login worked fine in IE and FF except for one persons IE browser wasn't accepting the cookie to expire in 60 minutes. All firewalls, adware stuff etc turned off. So what was done is instead of having a timed cookie the cookie is not set to delete after the browser is closed. This works fine for most people logging in to the script on their server. One person though has the script and works fine for about 40 minutes and then tries to login again and gets the following error. Warning: Cannot modify header information - headers already sent by (output started at /home/ldsands/public_html/player/[login to view URL]) in /home/ldsands/public_html/player/[login to view URL] on line 29 Warning: Cannot modify header information - headers already sent by (output started at /home/ldsands/public_html/player/[login to view URL]) in /home/ldsands/public_html/player/[login to view URL] on line 31 It seems the script works fine for a while then comes up with the error? This has happened 2 times now after I reinstalled the script. Need to fix this issue so it is compatible with all the newer versions of IE and FF Thanks