I have similar problems with Denon AVR-4310CI
1. The receiver was found by Twonky server without any problems. I could stream audio using the WEB-based user interface of the receiver. It was sluggish but worked for a while then broke for good. Now I can not access network menu. Tried to reset the receiver. No luck. It's unlikely that Twonky broke the WEB UI on the receiver but it's still a possibility.
2. I managed to push audio to the receiver via Twonky Manager but again there are various of bugs
a) Looks like Denon does not like capital letters in file extensions (e. g. MP3). It refused to play some of my files and accepted them after I changed the extensions to lower case
b) Same problem happens with long file names.
In both cases Twonky loses the device. Sometimes it finds it, sometimes I have to reboot the receiver
I had problems with MediaPlayer too. Looks like Denon DLNA support is a marketing trick. They could have done better for the money they ask for the device