This can mean the Denon doesn't support the format you're trying to send. Start by sending the common mp3 format, and PCM, and then work to other formats.
You can use Device Spy to determine the list of file formats the device supports. That requires installing Java and the Device Spy software.
Your device's manual indicates these formats are playable (WMA, MP3, WAV, MPEG-4 AAC, FLAC, and JPEG):
A server or server software compatible with distribution in the corresponding formats is required to play music files via a network. z1 Media server
• This unit is compatible with MP3 ID3-Tag (Ver. 2) standard.
• This unit can show the artwork that was embedded by using
MP3 ID3-Tag Ver. 2.3 or 2.4.
• This unit is compatible with WMA META tags.
• WAV format Quantization bit rate: 16 bits
• FLAC format Quantization bit rate: 16 or 24 bits
The device list is created by MC via receiving SSDP advertisements sent out by DLNA devices. You can see these announcements by going to Services & Plug-ins > Media Network.