Please try with guest additions 5.2.1 image [0] from https://www.virtualbox.org/wiki/Downloads
There was various issues [1] and for me that updated image fixed. It works on rc7 guest. [0] https://www.virtualbox.org/download/testcase/VBoxGuestAdditions_5.2.1-118918.iso [1] https://www.virtualbox.org/ticket/17163
