Hi All and @Jakub Dubovic,
I'm not the OP, but I did join this thread a long time ago, November 2020, complaining about this ongoing problem with Avast Premium blocking PC's on my internal network. I read your responses and the the FAQ you suggested. I just added a new PC on my network, and previously had followed all the steps gkinrade described earlier to diagnose the issue. This time, however, I set up a dual monitor and remote accessed into my server running Avast Premium, while operating File Manager on my new PC and viewing it on the other monitor. Using both monitors, I can see the actual block as it happens, and Avast logs the block as an IPv6 address instead of IPv4 (I never got an explanation about that, although I've asked it in this thread a few times). This time, as I had done previously, I entered IP addresses under Settings to exclude the block. It never worked completely before. I've entered an IPv4 range AND I've entered both the specific IPv4 and IPv6 addresses of the blocked new PC. Avast still blocked the specific PC. What DID work, surprisingly--and I did this on impulse just see what will happen, was I turned the "Enable SAMBA Protection" OFF on the Avast running on the server, tried to access the server through File Manager on the new PC (it worked) and THEN I turned "Enable SAMBA Protection" ON again on the server. Thereafter, the new PC was able to access my server. Go figure, right? We will see if this solves the problem.