UnitiServe Generated MP3 Files Will Not Play

When I ripped my CDs, I had the UnitiServe (US) set to store in FLAC format and to create MP3 320kb copies of the files, which I subsequently copied to my Sony Walkman MP3 player. Unfortunately, I have found that quite a number of the MP3 files generated by the US will not play on the Walkman, nor will they play using iTunes on my MacBook Air. I tried converting the FLAC files on the US to MP3 320kb using dBpoweramp and these play without any problem. Therefore my conclusion is that there may be a problem with the MP3 encoding on the US. Has anyone else experienced similar problems?

Having found that the MP3 files on the US cannot be relied upon, I now wish to delete them but cannot find any facility in the n-Serve app (both IOS or OS X versions) for doing this. Can the MP3 files be deleted safely from the LQ folder on the US using a network-connected computer?

It would have been nice if the US had an option of encoding to AAC format as an alternative to MP3.

Comments from Naim would be appreciated.