Jetpack Connection Broken

I’ve upgraded from 2.8.4 to 3.1.2. There were a number of issues, which are now largely resolved. But Jetpack continues to be problematic. I keep getting this error message in the stats block on the dashboard:

unknown_token: It looks like your Jetpack connection is broken. Try disconnecting from WordPress.com then reconnecting.

I’ve found no references to this issue on the web. (Except for a Greek translation in Automattic’s svn archive.)

So, I tried logging in and out of WordPress.com. I also cleared out the cookies in my blog and in wordpress.com. All to no avail.

Has anybody seen this error? Any idea how to resolve this?

Thanks for your help.