General Support: Warning: Cannot modify header information - headers already sent by (output started....

When trying to access any admin functions, or leave comments on a picture I get

"Warning: Cannot modify header information - headers already sent by (output started..."

They are caused by php lines such as:
header("Content-Type: text/html; charset=utf-8");

-Occasionally it is possible to comment out the header line and the warning goes away.
-When in the admin functions, however, and trying to browse Feedback all I get is this warning. Commenting out the header line
just causes the feedback page to be blank.
-When adding a comment to a picture I also get this warning and a blank page

Why have these header issues suddenly started appearing?
Is there a resolution to this problem?

Thanks,
justin1186