While investigating the issue I recognized a strange behaviour. The user
group is allowed to remove seed members once they were added but only if
they do not log themselfes out and login again. In other words, I add a
seed member with user A and remove it and it works. When I add a seed
member with user A, logout and login with the same user A again, it is
not possible to remove the seed member from the delivery and I get the
error message from above.