Hi,
I looked at this discussion when I started to have the error and i made me feel better that it wasn't only me who was struggling. I called apple support because my laptop is still new and so it is still free at the moment and told them about the error and that I wasn't the only one who was experiencing it. They did give me a few helpful tips how to work around it, but couldn't fix my original problem.
First if it is not working in one library create another one:
Shut down iTunes completly
Hold Option and click on iTunes icon on your computer
Click create new album
Call it whatever you want
Then there should be a new library with no songs or anything in it.
Go to iTunes match and start the iTunes match
All your songs should be there and start downloading
The error hopefully wont come up again....
This worked for me, I'm not too sure how everyone else will go. They are monitoring my account to see if it happens again. From the sounds of it she said that they had never had this error reported before so I don't think all those emails to apple support worked.
They are looking into it and i guess they will sort it out, otherwise no one will buy it!
HOPE THIS HELPS.... chels