Trying to remove certain users from certain groups raises and redmine error.

i.e.: deleting "foo" from group "bar" (using Administration->Groups)
leads to the following trace in production.log:

Started DELETE "/redmine/groups/207/users/211" for 192.168.122.1 at 2014-07-16 18:36:28 +0200
Processing by GroupsController#remove_user as JS
Parameters: {"id"=>"207", "user_id"=>"211"}
Current user: xxx (id=608)
Completed 500 Internal Server Error in 21.8ms

NoMethodError (undefined method id' for nil:NilClass):
app/models/member.rb:87:inset_issue_category_nil'
app/models/member_role.rb:42:in remove_member_if_empty'
app/models/group.rb:68:inblock in user_removed'
app/models/group.rb:67:in user_removed'
app/controllers/groups_controller.rb:103:inremove_user'

Some users were successfully deleted, but some others consistently raise this error.