So I was having the same troubles with all of the Discovery Channels not authorizing on TV Everywhere through my Philo account. Went through all the steps above deleting the source, rescanning, deleting the file & rescanning again, turning off VPN etc, but nothing worked.
By accident, I clicked under the Sources, then the # of channels right below Philo in the source list. It brought up a list of TVE channels. I clicked Scan all the way to the right of the TVE Channels in the header and it populated a list showing some with checkmarks and others like the Discovery Channels showing no login form found.
I then clicked on the gear box to the right of each individual channel, then selected Rescan for that channel. It sat there for roughly 30 seconds, then lo and behold a checkmark popped up next to it. I went down the list and did each one individually. Now, all the Discovery channels are working again!
Don't understand why rescanning all the channels together wouldn't authorize them, but doing them 1 at a time did.