Yes, the checkbox was set.
On the Graphene forum sombody had the answer. The google sdk pakkage in Ubuntu is an to old version. I updated the package and got the bootloader unlocked again.
Problem solved and installed GrapheneOS.
Maybe I go back to e/os but I must admit that installing Graphene was a verry easy process. I had e/os on my FP4 and found installing a bit of a drag. Switched to Ubuntu Touch on that phone witch is slidly more complicated I agree, but is I think, more safe.
Thanks for the help!
EJ