Plex client on iPad2 working but not working on iPad Air 2 for flac files

I'm running the following Plex server version on my iMac : Version 1.1.4.2757

I have an iPad Air 2 with iOS 10.0.2 (14A456) and running Plex app version 4.4. This combination is playing video files and mp3 audio files but refuses to play my .FLAC files which my music mainly consists of.

I also have an older iPad2 with iOS 9.3.2 (13F69) and running Plex app version 4.2. Good that I don't have been upgrading but iOs and Plex as this combination is still able to work flawless, including playing my .FLAC audio files.

So the questions are :

• is the Plex app version 4.4 causing the error causing the problem with .flac files ?
• is the iOS 10.0.2 (14A456) causing the error causing the problem with .flac files ?
• is a combination of iOS 10.0.2 and Plex app 4.4 causing the problem with .flac files ?

Note : if on the iPad2 I choose the iPad Air 2 as the player it has the same problem ...

Any advice from anybody ? :((