First, I would check at http://billing.microsoft.com to make sure that your OneCare subscription is indeed showing as active with a future expiration date. Then, check the PC date and time to make sure that it is correct. If OneCare continues to claim that you are using a trial, you should have a link to subscribe/activate in the main OneCare window. If you do not, then I would think that the PC date and time are incorrect, however, if the copy of OneCare was previously activated, it should not revert to trial mode unless you performed a reinstall or perhaps a System Restore. If the problem persists, particularly with the initial mystery error, I think it may be worth uninstalling and reinstalling once you've confirmed your subscription status. You can also contact support for help.
How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
If it fails to validate your subscription, select the option that you are using a trial or beta copy and you can proceed to email support without validation once you've signed in.
-steve
