It says in the manual:
Charge the device
If failure continues, switch off and do not use
Device must be charge. If the message still there after recharge, contact Dräger.
this error points to the battery not charging, not holding charge or the battery pcb failing to sense the battery voltage.
I had this last week, checked the battery 2.5vdc(2.4vdc is desired), placed it intot he charging cradle and updated the date, for 30+ minutes, no change. so it's the battery pcb.