please write the list of active plugins and themes

Already did:

default theme (2020). Every other plugin disabled but CSPC and WooCommerce

EDIT: Mind have reported a separate issue of latest WooCommerce and headers, but it doesn’t seem to directly concern CSPC.

Hi Salvatore, Saumya,

@isaumya:

Yes I am also seeing this error over and over again and not just on line no 43 but on multiple lines.

You reported that same issue here. How did you fix it?

My mu-plugins has only this: health-check-troubleshooting-mode.php which is already std WP. Getting rid of it anyway doesn’t help.

I still get this stuff filling up my debug.log:

PHP Warning:  Cannot modify header information - headers already sent in /wp-content/advanced-cache.php on line 38
PHP Warning:  Cannot modify header information - headers already sent in /wp-content/advanced-cache.php on line 41
PHP Warning:  Cannot modify header information - headers already sent in /wp-content/advanced-cache.php on line 42
PHP Warning:  Cannot modify header information - headers already sent in /wp-content/advanced-cache.php on line 43
PHP Warning:  Cannot modify header information - headers already sent in /wp-content/advanced-cache.php on line 44
PHP Warning:  Cannot modify header information - headers already sent in /wp-content/advanced-cache.php on line 45
PHP Warning:  Cannot modify header information - headers already sent in /wp-content/advanced-cache.php on line 46
PHP Warning:  Cannot modify header information - headers already sent in /wp-content/advanced-cache.php on line 47
PHP Warning:  Cannot modify header information - headers already sent in /wp-content/advanced-cache.php on line 48

@isaumya: I shouldn’t have any Number 2, as tested and reported in the OP:

Switched to default theme (2020). Every other plugin disabled but CSPC and WooCommerce.

Any other suggestions?

Thanks.

Okay, did some more tests setting this catcher into advanced-cache.php

if (headers_sent()) {
error_log( var_export( headers_list(), true ) );
}

and got this:

array (
  0 => 'Expires: Wed, 11 Jan 1984 05:00:00 GMT',
  1 => 'Cache-Control: no-cache, must-revalidate, max-age=0',
  2 => 'Content-type: text/html; charset=UTF-8',
)

First 2 entries refer to WP core, specifically to wp-cron.php line 21:

/* Don't make the request block till we finish, if possible. */
if ( function_exists( 'fastcgi_finish_request' ) && version_compare( phpversion(), '7.0.16', '>=' ) ) {
	if ( ! headers_sent() ) {
		header( 'Expires: Wed, 11 Jan 1984 05:00:02 GMT' );
		header( 'Cache-Control: no-cache, must-revalidate, max-age=0' );
	}

	fastcgi_finish_request();
}

Couldn’t figure out where 'Content-type: text/html; charset=UTF-8' is coming from though.

The issue seems to arise from my cron job as it isn’t WP’s, but invoked by Linux. In wp-config.php I’ve:

/** To use Linux crontab, make sure WP Cron is disabled */
define('DISABLE_WP_CRON', true);

The fix that seems to address the issue is adding this line to advanced-cache.php before setting any headers:

if( defined( 'DOING_CRON' ) ) die();

if( intval($swcfpc_config["cf_maxage"]) > 0 ) {
    header_remove('Set-Cookie');
}

Thanks for confirming, Saumya.

I already tried taking out health-check-troubleshooting-mode.php from mu-plugins, and later also tried your suggestion of cleaning up redundant symbols from the end of wp-config.php but neither of those helped in my case.