Agent(s) appear offline in the protection console. MMS service is running on the machine and there are no connection issues.
Even when the agent is shown as offline, backups run correctly.
Issue in the product.
This issue is fixed in Acronis Cyber Cloud 21.04.
As a workaround for earlier versions, stop MMS service on the agent for 10 minutes, then start it back.
If you are facing this issue with Virtual Appliance, stop the Virtual Appliance for 10 minutes, and then start it again.