Hi Hendrik
I am not sure. The Login process is clearly handled by Paypal, not by Events Booking. So if you enter the correct account, you should be able to Login. If you are sure that you entered correct account and could not login, maybe you will need to call Paypal to ask about it as it is not controlled by Events Booking.
Maybe it doesn't work is because your account is Live but you are testing Events Booking in test mode ?
Tuan