I have had that issue as well. Login Agent failure seems to be completely random. I couldn't replicate that issue. There's no CTRL+ALT+DEL on our runtime resources. As Jerin mentioned, there could be windows update popup banner which could've been the issue, as there was a timeout. But still, when I tried to login manually during that time, I couldn't find any windows update popups.
It just started working after a brief hiatus.