I can reproduce following behaviour:
Windows 10 Pro x64, 1703 version. Current Avast Business Management Console, Antivirus Pro Plus test license. Installed the Agent on the very same machine.
If I disable the firewall of the client too late, the Web Front End is not reachable, connection refused. When I disable the firewall early enough, the interface is reachable. I can only log-in with Firefox or Chrome. Edge/Internet Explorer will work for the first account setup, but log-in always returns to the login web site with the Microsoft web browsers.
When the firewall is disabled, the Avast client will crash after a few minutes. It shows
.. something along the lines "Avast has failed" in german. It offers to restart the service, which will show green light for a few seconds, but then fail again.
*Update 13:20h* The client seems to crash reliably even without disabling the firewall!
Program version: 17.6.2525 (build 17.6.3625.0)
VIrus DB: 140904-2
Please let me know if and how I can help gathering data for debugging or help tracking down the issue.