I had issues with flac files that I created. Every other player I own played them just fine except for the DP-S1. Turns out I didn't understand that flac had its own tagging and I didn't need to add id3 tags which was why they were flagged as incorrect. The other players didn't care, the Onkyo did. Easily corrected once knew I did something wrong by just removing the id3 tags.
I have a feeling something similar is happening here. My tablet and other mp3 players handle the file just fine. The Onkyo lists it if I use the file browser but when I try to play it I get the invalid file error. Then it no longer shows in the browser which I thought was an interesting behavior. I'm guessing that again its a case of the extracted aac audio is slightly incorrect or out of spec and whoever coded the DP-S1 was very finicky in requiring the format for files to follow spec.
It was ripped from a YouTube video, well demuxed to be exact. Just extracted the stream and saved it to a file. I used TubeMate, an Android app, to extract. I'll give another app a try. Perhaps I'll move to my Mac and use ffmpeg to demux. After all I don't need conversion.