Is your firewall network profilen in puplic or private mode?
If not already done, try setting it to private and reboot ..... did that work?
Setting up Firewall Application Rules in Avast Antivirushttps://support.avast.com/en-eu/article/Antivirus-Firewall-Application-Rules=================================================================
NOTE:The Network profiles control the settings which determine how strictly Firewall protects your PC.
Private: a lower level of security suitable for when your PC is connected to a trusted network such as your home or work network.
This profile enables better connectivity,
and permits all communication within the network.
Public: a higher level of security suitable for when your PC is connected to a public network such as in a cafe or airport.
Because public networks present greater security risks,
no incoming communication is permitted when this profile is set.=================================================================
Also see info posted here by @BWith https://forum.avast.com/index.php?topic=227130.msg1505540#msg1505540