I've set up a new MongoDB single node replicaset using MongoDB 4.2.7.    Although I'm using the same server and agents configuration settings as other replicasets in the same deployment - there is a regular MongoDB alert being triggered. 

Backup is down
Backup Agent

Alert Details

CREATED 2020/05/29 13:12 GMT
DATA-BEARING HOSTS (INSTALL AGENT ON AT LEAST ONE) rsdevelop (at least one of the following mongod(s) needs a co-located agent: [muserver.xxxxx.net:27017])
OPEN

The scheduled backups are being completed , and there aren't any errors. Why is the Backup is down  alert being triggered?

I checked through the MongoDB documentation and the related text for this alert is:

In the MongoDB documentation there is a specification about how MongoDB alert notifications works:

"When a condition triggers an alert, you receive the alert at regular intervals until the alert resolves or Ops Manager cancels it. You can acknowledge an alert for a period of time, but if the alert condition persists, you will again receive notifications once the acknowledgment period ends"

I followed the path to the Backup log file on the host and reviewed all messages  - with no error messages - apart from the initial one.

So I started reviewing the Backup Agent status of the specific host - just doublecking the status and any other  observations. I noticed the host Backup Agent was in standby , and the other host in the deployment was Active. But remember reading in the MongoDB documentation, suggesting it's not a problem:

"Only one host can backup a deployment at a time. On the Server tab, the host that is backing up the deployment displays Backup - active. Any other host with Backup activated displays Backup - standby"

Next step was to restart the automation agent for the MongoDB server. 

--Stop the Automation Agent 

sudo service mongodb-mms-automation-agent stop

--Start the Automation Agent 

sudo service mongodb-mms-automation-agent start

The restart of the Automation agent - did not resolve the issue. I went to the next step , and restarted the MongoDB server - this stopped the alert reappearing. The Backup Agent is working fine , all snapshots and oplog slices are successfully and up to date.