Immediately after login into EAC/ECP I'm presented with 503 error. However, simply refreshing the page brings up EAC as expected and it works fine after that. Until I log out and attempt to log in again after a while, the story repeats itself.
Even ID 1003 from MSExchange Front End HTTP Proxy and 1309 from ASP.NET 4
I tried using both the self-gen and a CA issued cert on the back-end site, makes no difference. Just did that to cover bases others were on but I didn't think this would be it since it's such a temporary issue, simply refreshing the page brings it up, which a cert problem would not present.
Similarly, opening the EMS on the server initially throws up pages of red errors but when closed and opened again, it start fine.
So something seems to be pausing/stopping and needs a few seconds to start back up.