Canon 5D2 / 5d Mark 2 Error 20

Back in 2009 I posted about the error 20 that I had experienced on my Canon 5D2

Since then I have taken the camera to be checked by my local camera specialist, Miles Whitehead, who arranged for the error codes to be checked and cleared. Since the codes were cleared from memory the error 20 message has not reappeared in around 6 months so I'm hoping that this is now resolved.

Canon 5D Mark 2 (mk II) Error 20 Shutter Mechanism

I had error 20 last night on a shoot. This is the first time it has happened. Tried removing battery but made no difference to error.

Camera appeared fine until I took a photo and it then locked up. Strangely Live view was working fine and could shoot video and stills ok. Battery was showing as being over 50% full.
When I got home I put the battery on charge. It started at flashing 1 light for a long time, seeming to indicate that the charge was very low and then slowly went through the sequence of flashes until it was fully charged.
Put battery back in camera and all seems to be ok now so I'm hoping that it is an error reporting charge on battery and not a camera fault as it is just over 6 months old.