mbsync :Error: channel :<channel-name>-remote:<folder>-:<channel-name>-local:<folder> is locked
This happens when there is another mbsync instance running in parallel and it has taken a lock on the particular folder. In my case, I am running mbsync via a cron-job, and it tends to leave dead instances around if the laptop sleeps during a run. Kill all instances of mbsync and restart the process and you should be fine!