Recompile Virtualbox kernel modual
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sudo apt-get update
sudo apt-get install build-essential gcc make perl dkms
sudo /sbin/vboxconfig

(oh, and I probably should restart the pc after recompile.)
have to do this with every kernel upgrade (including small updates)
(should have virtualbox closed before doing this)
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Found on August 10th, 2019 @ 00:08 (past midnight)
on:  https://askubuntu.com/questions/1035030/virtualbox-guest-additions-installation-problem