If anyone is experiencing the same:
delete security ipsec policy ipsec-policy-external perfect-forward-secrecy
I had perfect-forward-secrecy keys group5 active - however pfsense and juniper seem to "dislike" pfs - after disabling pfs everything started immideatly to work. We recreated this a couple of times and could reproduce this.
Thank you all very very much for all your help.