I guess it is not just us.
I am here whilst the game is reloading after it crashed (not for the first time )when claiming the reward.
It used to be possible to claim several at once,important when the time is nearly up, now you have to leave the claim page then return there, and then reload when it crashes.
I am wondering if what you are describing is lag in the system or something else.
I would do as you suggest, and do not be in a hurry to click start.
Another thing to consider is that the start times of events and bonuses are no longer consistant and may start some time later such that it benefits to wait till they have actually started.
Also I believe it may still be necessary to log out and back in to make sure the bonuses have registered ,otherwise any new bonus could be lost.