I did this, but that didnt fix it. The year field had the year corretly displayed in Tag&Rename. Nontheless, I deleted it and re-entered it, then re-imported into Sonos and rescanned the library.
But the error remains the same. "unsupported format"...
Ripped a CD in EAC to FLAC, works fine there, so it definitely seems to be a MC13 problem.