Could different timezones be responsible for this strange behaviour?
I have just an idea:
In debug-log I've found this part for my event:
My original event starts at 06:40 o'clock.
So originally it might be stored as 08:40+02:00!?
Then if I acknowledge the popup window it might lose the information about the timezone +02:00 and stores only 08:40 (without timezone information) and therefore it is thought to be in UTC+00:00 instead of UTC+02:00. So in google-calendar it is stored as 08:40+00:00 which in my timezone will be 10:40+02:00.
Next time the event is read with 10:40+02:00 and saved with 10:40+00:00. And so on and so on.