Cannot modify header information All in One SEO Pack Troubleshooting Forum

Thank you in advance for your help.

I am running into an error when this SEO plugin is activated. When adding a 'www' to the begining of the URL I get the error:
"Warning: Cannot modify header information – headers already sent by…"

Also when I activate this plug-in WP gives me this error message:
"The plugin generated 1 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin."

I am using Twenty Ten on WP 3.1
I read the other posts about deactivating other plug-ins. I have done this and now it is the only installed plug-in, I still get the error.

Please help.

-Mike