Nucleus will sometimes show the following error:

Warning: Cannot add header information - headers already sent by (output started at /home/www/path/to/certain/nucleus/file.php:LINENUMBER) in /home/www/path/to/another/nucleus/php/file.php on line LINENUMBER

Almost always this is caused by whitespace at the beginning or the end of the php-file where the error occurs. It can be fixed by editing that file so there remains no more whitespace (or other data) before <? and after ?>.

If that doesn't fix your problem, you can try to identify the initial source of the by turning the Debug mode on. This can be done in globalfunctions.php, by changing $CONF['debug'] = 0; to $CONF['debug'] = 1;. Then try to reproduce the error again. With the information you then get, you can try to fix your problem. Or ask for more specific help in the [[Nucleus Forum]].