@Ninja I tried flushing the cache and disabling and re-enabling CloudFlare, with the same results.
@manhta I verified that this problem exists when the CloudFlare maximum is set to 200 MB as seen in the following image:
(Tested with chunk size of 150 MB.)
Since it works with the other upload form via the profile "My Videos" page but not via the activity stream postbox, doesn't this already exclude CloudFlare as the problem?