This occurs when Veeam Agent for Microsoft Windows is initially installed on a Windows machine in Standalone operation mode and then the machine gets added to a Protection Group, which switches it to Managed operation mode. Even after the switch to Managed mode, the autorun registry entry to start the Veeam Endpoint Tray persists, causing the OS to attempt to launch the agent after user login. However, Veeam Agent for Microsoft Windows cannot be controlled directly from the Windows machine while operating in Managed mode.
Solution
To prevent this from occuring, remove the registry value named Veeam.EndPoint.Tray.exe from: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\
The following PowerShell command can be used to remove the registry value:
Remove-ItemProperty -Path "HKLM:\Software\Microsoft\Windows\CurrentVersion\Run" -Name "Veeam.EndPoint.Tray.exe"
https://www.veeam.com/kb4565