I have been importing quite a few songs into iTunes v 10.7 (21) on OS X 10.8.2 and this error is popping up when the upload process has a hiccup. Even if you are not currently in a state when you would expect uploading to the iTunes Match cloud, the software can run a sync and throw this error.
There's nothing you can normally do on your end other than disconnect from the internet and wait for the servers to clear up. In my case, no action was needed to resume uploads. The iTunes process was patient and resumed once a short timeout passed.