The ‘Unable to acquire lock’ message is actually not really an error but more like an info message. This message is logged in case a background task is started when a different instance is already running. The started background task checks is a different instance is running and logs this message and exits if it sees another instance.
So, to be a bit more specific, if you let cron start the background task once per minute and the task runs longer for one minute, you will see this message. The task can run longer than one minute if it has many emails to send or fails to send emails (and times out while doing so), for example.