FireFly Media Server (formerly mt-daapd) › Firefly Media Server Forums › Firefly Media Server › Add-on Software › A standalone Java client: FireflyClient › Reply To: A standalone Java client: FireflyClient
The line of text next to the red circle with the white cross says all you need to know.
“Unsupported audio encoding”
In other words, unsupported file type.
At a guess I’d say you were trying to play an aac file (.m4a)