I have also had a problem with the Avast Firewall blocking access to internal Routers, WiFi APs and servers that use a browser on port 80 to log in.
Turning off Network shield does not fix the problem
Turning off Firewall does fix the problem.
Have added all the devices as 'Friends' to no avail.
Added an Intranet entry to Packet Rules to allow anything in/out for 'Friends' addresses to no avail.
Have now turned off the Automatic Port Scan and these does appear to sort the problem.
***********---------------------------UPDATE------------------------***************
Following the recent update to version 17.4.2294 (build 17.4.3482.0) the problem is back.
I can momentarily log into a device on my internal network then within a minute it becomes unavailable along with every other internal web address. I can still browse and log into external websites but nothing internally.
I disable Avast IS and everything works.
Yes Automatic Port Scan Detection is disabled.
Yes Friends network is defined for the whole network range.
Also Opera is very slow to start.
And why I am on the subject does Avast insist on changing the internal network from Private to Public and back to Private when ever I login into my desktop PC with a static IP address on a hard wired ethernet connection
Yes Automatic network profile switching is disabled.