Can't retreive song title / artist / album using the Denon / Marantz Binding


I have managed to connect my Denon AVR-X2500H to openhab2 using the Denon / Marantz Binding.

I am now able to control volume, image input, …

But according to the documentation here:

you also can retreive the current played tracks name, artist and album.

My Itemsfile:

String denon_artist   "Artist"                       {channel="denonmarantz:avr:0005cacf1c36:general#artist"}
String denon_album    "Album"                        {channel="denonmarantz:avr:0005cacf1c36:general#album"}
String denon_track    "Track"                        {channel="denonmarantz:avr:0005cacf1c36:general#track"}

But when I add those items as “Dummy” to my Habpanel, I only see “NULL” as value.

The meusic is played through the Spotify connect feature from my mobile phone.
On the screen of the denon receiver, it says “Spotify”,
but on my TV, I can see the albums image and the title.

EDIT: I just tested playing MP3-Files from a USB-Drive.
There I can see the tracks name on my screen and on the display of the receiver, but the item still has “NULL” as value

Also, when I access or
to receive the image from the currently played album, I only get

Error 403: Forbidden
Access Forbidden

Am I missing a setting on my receiver?

Thanks in advance for your help.