"Cannot modify header information" when user A visits checkout pages from user B

1. User Alice completes order 29
2. User Bob visits /checkout/29/review or /checkout/29/complete
3. User Bob sees Drupals "Page not found"

Now the Drupal log lists the 404 (of course) and the following PHP warning (same minute) ("Location" is the visited checkout page):

Warning: Cannot modify header information - headers already sent by (output started at /…/includes/common.inc:2607) in drupal_send_headers() (line 1243 of /…/includes/bootstrap.inc).

I read (for example at #1129234: Cannot modify header information) that the cause could be a trailing blank line or PHP tag.
I wonder if this could be the cause (didn't find anything) for my case. These "headers already sent" warnings are only logged for those described cases where I visit completed checkout pages that are not from my own orders.

This is only happening for existent order IDs.

Could someone test if this is happening at his/her site, too?