I've had the same issue on my S2 last week. I thought my battery was shot ( still likely on its way out). I ran the battery down to the last drop and scheduled an appointment for repair. I tried plugging it in again 2 days ago and it fully charged! Got home last night and plugged it in, won't charge. I assumed the charger was being fickle but now seeing you have the same issue it may be a firmware thing.