I've detected this problem many times with the last version of iTunes and now with iTunes 11.
After matching all my songs, I went through many of them to add to a higher or change to a better/right resolution artwork. After doing this change and updating iTunes Match, it deleted all my changes and replaced the artwork with the first one i've used, I mean the old lower resolution artwork is back. I got so frustrating.
Every time I get thath error, so I start the process over and over again... And the same error repeat.
Is there a problem with the Apples Servers or ?
Do I have to change something in my iTunes ou Mac?