Does someone knows why H-IIB rocket can stay fuelled on launchpad for many minutes, just venting excess gaseous H2 and O2, while Ariane 5 must be connected to umbilicals almost to the very last moment before lift-off?
Payload margins most likely. The reason to stay connected is to stay topped up to the last second, to have the most available fuel for the mission.
HII possibly has sufficient margin for the flight even with venting.
According to the H-IIA user manual (PDF page 219, figure 5.7.12), in a normal launch, propellant loading stops at 3 hours before launch.
But there is an option ("98% additional loading") to continue propellant loading until a few minutes before liftoff. So Geoff's answer was correct.