Headers already sent I found this error repeated several times for two of the pages on my website.
    Warning: Cannot modify header information - headers already sent by (output started at 
/home/customer/www/mydomain.com/public_html/includes/common.inc:2800) in user_cookie_save() (line 4041 of 
/home/customer/www/mydomain.com/public_html/modules/user/user.module).

I checked a bit around and found other similar errors with &quot;Headers already sent&quot; but none of them on the user_cookie_save function.
How can I debug fix it?
I'm running on Drupal 7.78, PHP 7.3.2.7 and as modules that could influence the cookies, I'm running just the EU Cookie compliance module.
What I find strange, is that I do not recall having seen this error before. There is a small chance that the error was there ad I did not see it because the logs are filled with page not found, and access denied errors by nice persons from the internet :) I would say though that that is a small chance as I've checked many times the logs in the past and did not see this error.
I have no idea what changed, and also no idea of what is the impact of this error, it's a warning, but I can't reproduce it.
Many thanks
HTML requests return headers, that tell what the response will be, and the body, that is the response. The headers are required by browsers to understand how to interpret the data that is in the body, therefore no output must be sent before the headers. This error is outputted when headers are sent after some body output has been sent. When working on a Drupal system, there are three common causes to this error that you can debug:

An echo statement in a module/theme (and not in a template). Sometimes developers will echo out a value for debugging purposes, and forget to remove that line.

Blank space before the opening &lt;?php tag in a document on the system. This is usually in a custom module or theme, or can be in settings.php, or settings.local.php.

A closing `?&gt;' tag, with whitespace after it. Drupal coding standards are to not use the closing php tag, so if you find this anywhere outside of a template, you can probably remove the closing tag. At the very least, you should ensure there is NO whitespace after the tag.