I upgraded VBoxGuestAdditions to version 6.1.4 on my virtual machines and notice that shared clipboard doesn't work. It was working before the upgrade.
Solution
There is bug in VBoxGuestAdditions_6.1.4.iso. For details, see bug at https://www.virtualbox.org/ticket/19336. I simply downgrade back to VBoxGuestAdditions_6.1.2.iso.
About the author
Xuan Ngo is the founder of OpenWritings.net. He currently lives in Montreal, Canada. He loves to write about programming and open source subjects.