HDCP issues should largely be non-existent at this point. HDCP has been around for a good number of years by now, and it is far more likely that the issue you are running into is a cable issue. Since the cable itself uses a transmission side that draws power from the Denon receiver output, it is feasible that the Denon is not putting out the proper voltage necessary to run the transmitter at full power. It may be as simple as needing to put in a powered amplifier, which would be included in the 1x2 splitter listed above, but that may not solve things.
The hard part is that there is no easy way to diagnose this type of issue because there are any number of points of failure.
The culprit is almost definitely the Denon in conjunction with that specific cable.
The problem with that being the issue, is that you aren't getting that Denon fixed. You will have to fix the cable.
It is worth saying that I have installed a half dozen of the 2312 receivers last year with 25+ lengths of HDMI cable and every single one of them performed flawlessly.
This isn't at all what you are hoping to hear, but if you do have access to pull a new cable, then I would go down that pathway.
Pick up a 50' 22awg cable from Monoprice and a 50' CAT-6 cable from them and drop them down the wall. Not any fun in the attic, but at least the weather is cooling down some. And, once those two cables are in place, you should be good for a very long time.
Oh, and just for safety - I would put in at least 2 if not 3 pieces of cat-6. Cat cabling is getting utilized more and more frequently, and HD-Base-T is an unbelievable cabling standard, which hopefully will work it's way into consumer products in the next few years and eventually replace the HDMI cable completely.
