One of the more common problems that is the result of a whitespace at the end of a file.

The error looks like this:

Warning: Cannot modify header information – headers already sent by (output started at /home/user/public_html/config.php:40) in /home/user/public_html/index.php on line 6

In order to sort it, remove any extra lines from the file specified in the particular error message.

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