ok I see. Could you show this output?
show vlans klient1 extensive
One thing i would like to see from your test.
remove or deactive the irb. then ping host 1 on klient1 from host on klient2
activate the irb and repeat the same. i am really more curious now about this els. I don't have any such systems to test so I have to rely on the efforts of you and others with such experience.