Seeing Lock already exists: /var/run/munin/munin-update.lock? Don’t worry it just means your munin updater is slow
Luckily this forum post explains the issue.
I’ve had this problem if the munin-update process takes longer than 5
minutes. The reason this happens is because the cronjob is set to run
every 5 minutes to gather data, so if it is still running from the last
job, you will get this error.