This is not an error. This is specific to the Mac or iOS client. The error could be due to the VM idle timeout policy applied to the pool, which disconnects idle VMs after the designated period. The user can try relaunching the desktop, and they should successfully reconnect to the session.
This error code is the same as 3(3) for the Windows client. Please review the Control Timeout policy or Group polices:
Computer Configuration → Administrative Templates → Windows Components → Remote Desktop Services → Remote Desktop Session Host → Session Time Limits
User Configuration → Administrative Templates → Windows Components → Remote Desktop Services → Remote Desktop Session Host → Session Time Limits