it's terrible, again, something does not work anymore.
Now I can create parentevents and the children'events are created automatically. BUT now registration for a childrenevent is no longer possible.
thanks for the quick help!
Now I hope I have the right settings to finish my project. If someone has the same problem, here is Tuans answer that solved my problem.
"Look like the error happens because these events have same alias with other events created in the past. I went to Events Booking -> Configuration, look at SEF router settings tab, set Insert Event ID config option to Yes"