Stop the unifi-video service

sudo service unifi-video stop

Delete the journal with

rm -rf /usr/lib/unifi-video/data/db/journal/*

Switch to the unifi-video user

sudo su unifi-video

Repair the database

mongod --dbpath /var/lib/unifi-video/db --repair

Exit the user

exit

Start the unifi-video service

service unifi-video start