Guest additions stopped working in VirtualBox 6.1

Question

I have the same problem as here. Nothing from answers helped. I tried http://youtu.be/5_fluJuJc6E?t=58 but Remove Attachment is grayed out. I have VirtualBox 6.1 and Lubuntu 18.04.

Answer 1

This answer helped.

sudo mount -o loop /tmp/VBoxGuestAdditions_6.1.26.iso /mnt

and

sudo sh /mnt/VBoxLinuxAdditions.run