What version of the Horizon Client do your end-users have installed?
Am I reading your post correct that any HTML access is working ok but Client isn't unless you disable H.264 in the properties?
I have not seen behavior like this to date with customers or labs but wanted to check versions and help further.
We are using the newest client version 8
It sounds like you have some ports blocked on some segments of your environment. I wonder if the H.264 Decoding might be a coincidence.