Note that the regular error logs only show errors from Apache, not PHP. So, those ModSecurity messages are from an Apache security rule that DreamHost setup to protect all web sites. It may just be a coincidence that they appear 40 minutes before the failed upload, although they could indicate that attacks are bogging down WordPress.
Premature end of script headers: async-upload.php" means that Apache tried to run WordPress's upload PHP, but PHP unexpectedly failed. Apache doesn't know the exact reason PHP failed -- could be file size limits, permissions, process overload, etc, etc.
To record PHP errors (if there are any), you'd need to configure PHP logging via the
php.rc) file. This is the configuration I usually use for production sites (replace
user with your user-name):
display_errors = 0
error_reporting = E_ALL | E_STRICT
log_errors = 1
error_log = "/home/user/php.log"
display_error to 1 will make PHP show errors in the browser, which is useful for live debugging.