This happened to me after I upgraded to WM5. To fix this, there is a 2-step process. First, put the unit in the charger. Once you turn the unit on, disable any and all Today Screen plugins that moniter Battery.
This is a symptom of a dead battery. I wish they had a better screen to denote that other than the "memory error" screen that comes up. There is a large write-up and discussion on this on the
http://www.aximsite.com forums. Good luck.