I am currently using Sophos UTM 9 and have created an exception for the following sites within Web Protection > Filtering Options
The same issue occurs when using 2017.1 as well.
As soon as i turn off a proxy on a client computer, access goes through fine and the sign in box appears.
With the proxy on, the log throws up the following:
Trying to execute the disabled command com.esri.agol.commands.signInCommand (NotEnabledException) - [main]
with the com.esri.agol plug-in at fault
Because of this, i also can't access the Python Console as that requires internet access as well
Any ideas anyone?