Hey sk8erboy! I just sent you an email!
Actually, I'm talking about the backup battery.

The backup battery is a small, non-removable battery inside the phone, which is by the phone to remember the date and time!
My thoughts would be that this battery may have run out of juice.
Interesting point with hot-swapping batteries! I didn't know about that!

And thanks to all other people answering to this thread! I didn't know that this was such a widespread problem!
I also tried to see whether it mattered if I force shutdown or use the powerbutton! To me it makes no difference!