WPMU DEV Update Notifications plugin keeping losing my API key

I have a current subscription to WPMU DEV, and my Update Notifications plugin is current (2.0.1). WordPress is also current (3.1). This is a multisite, multi-db setup.

When I enter the API key for WPMU DEV in the Update Notifications page and click Save Changes, it's saved -- the field turns from red to green, and all the updates are displayed.

But if I then leave the page and come back, or even if I refresh, the API key field is reset to empty (red) and the updates aren't shown. I have to re-enter the API key again. And then again, it's lost when I leave the page.

I didn't see any other threads for this problem, but it seems like something that should affect anyone with a subscription. Ideas on why the field is resetting?

Thanks very much.