1. You installed Acronis Cyber Backup 12.5 Update 4 (build 13400 and higher),
2. When you navigate to Settings -> Agents -> Details, you see that Agent is shown offline since year 1754 (exact date depends on your environment). You cannot run a backup plan on this machine or apply new backup plans to it.
Issue in the product.
The issue has been fixed in Update 5 (Build 16180), please update to the latest build.
To work around the issue without updating, do the following: