255% is typically when the Edge is not in communication with the Di2. Check that the Di2 is actually in communication with the Edge. Change gear position and see if that registers on the Edge, then check the battery value.
Mine hasn't been working, showing bogus charge levels when riding, even when it is in communication (i.e. showing selected gears). I get 255% charge and have also gotten <20% charge when fully charged. The first time I had the latter, I believed it and did a 30 mile ride with as few shifts as possible.
Since then I have discovered a hard reboot usually clears/corrects it.
