
Add the following to your /etc/rkhunter.conf file
#Config to add byobu files to your allow list
ALLOWHIDDENDIR=/dev/shm/byobu-*-????????/.last.tmux
ALLOWDEVFILE=/dev/shm/byobu-*-????????/.last.tmux/*
ALLOWDEVFILE=/dev/shm/byobu-*-????????/*/*
ALLOWDEVFILE=/dev/shm/byobu-*-????????/*
ALLOWDEVFILE=/dev/shm/sem.*
Ref: https://bugs.launchpad.net/ubuntu/+source/rkhunter/+bug/1776632