When I check watchdog, I have lots of PHP warnings like this:
Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /srv/bindings/9c42927e5316467d82a6fb1be50ed34e/code/includes/file.inc:1972) in drupal_session_start() (line 287 of /srv/bindings/9c42927e5316467d82a6fb1be50ed34e/code/includes/session.inc).
There are a lot of reports of errors like this, apparently most of which are caused by having spaces at the beginning or ends of PHP files. But in this case, I think the problem is related to file uploads for the following reason. The Location of these warnings are paths like this:
The Referrer is my "manage pictures" page, which allows users to add a Picture content type which includes an Image field.
So, it looks like session_start() is having trouble during uploads. Is there a standard fix fox this kind of problem?