I had a similar (but not identical) problem a few days ago. I had a couple of tracks that were nominally .mp3 tracks but my MP3 player kept saying that the digital rights needed renewing (even though I had bought them only the day before from a DRM-free site).
Even though they already had .mp3 extensions, I ran each track through the WMP Windows Audio Converter, set to output as MP3. After that, they played perfectly.
It might be worth trying this.