What software version is your MCU? Have you tried the Hybrid content mode on the MCU, you might have to upgrade your MCU to I believe one of the 4.x software releases.
- Passthrough: The MCU does not transcode the content before transmission. The data is simply repackaged then sent to endpoints that support the original codec.
- Hybrid: The MCU sends a passthrough stream to eligible endpoints, and a transcoded stream to other endpoints that advertise support for the Outgoing transcoded codec.
- Transcoded: The MCU decodes the incoming stream and re-encodes it using the Outgoing transcoded codec, before sending it to all endpoints that support the Outgoing transcoded codec.
Can you tell us more about how your C40 is configured, have you done any configuration on it since you've installed it? Such as the conference video bandwidth on how the codec will handle bandwidth when content is being sent, or anything else of the like? Since you said you're using the HDMI input for your content, have you confiured the HDMI 2 input to use the quality "Sharpness" and use the type "PC". Have you confirmed the resolution that the C40 is getting really 1080p?
How do you have your MCU configured, have you done any configuration on it in regards to content settings, or bandwidth?
Most of the time the reason the resolution drop is due to not enough bandwidth, the main video channel has to share with the content channel. However since you've tried to increase your bandwidth beyond what should work, it sounds like a configuration somewhere, possibly on the C40, since you said an MXP codec can connect and transmit the correct resolution just fine. Have you tried a point-to-point call yet between the codecs, to remove your MCU from the mix, that might help to determine if it's the C40 or not.