The reason for this error is because the UTM is in the process of downloading email and is shut down and the download is in process. The lock file does not get cleaned.
You can fix this by simply deleting the lock file.
Follow these steps:
SSH into the UTM
Delete the lock file - it is normally named after the upstream UUCP System name so in this case netactive. So there should be a file called LCK..netactive that can be deleted.
Once deleted, the UTM will resume downloading emails.