I have the same issue. Contacted LG support in the U.K. As usual, the automatic response was first to blame incompatibility with the Denon Receiver and suggest I test my internet speed. They also want me to plug the devices directly into the LG TV. This is not possible because my entire system has the AV equipment a distance from the TV and it’s all channeled through conduits in the wall and ceiling. So I’m unable to try this. Of course the “bypass” with the AVR turned off suggests it does work (see below). I copied a link of this discussion to the email I sent to them in the hope they would read it and also stressed that everything worked perfectly prior to this update. I am never hopeful of the technical support when the first reaction is not to read their emails properly and give knee-jerk (canned) responses.
I’ve tried HDMI bypass and it works if the AVR is switched off. But then of course I don’t get my surround sound. If the factory reset works, that may be the route out of the mess. Otherwise we rely on the goodwill of LG to fix this rotten update.
05.10.15 My setup is ATV > Denon 2600H > LG C9 (65 inch). did the OTA update and Dolby Vision is working fine.