Thank you for confirming.
This is a known issue at the moment, on many devices, and not isolated only to Denon.
Pandora made some changes to their server environment earlier this week, which has broken the compatibility with several devices.
For this to be resolved, Denon has to release a patch/update to bring your receiver back up to the compatibility standards required by pandora for connecting to their services.
You can periodically go into the settings menu on the denon under network/other setup and run the Firmware Update function, until one loads and service is restored.
Typically we expect to see this type of update happen within about 2 weeks. The same thing happened about a year ago and denon had a patch out in about 10 days.
In the meantime, you can continue using pandora through an external device such as streaming via a phone/tablet through airplay or bluetooth.