I have to disable the shield on the client side.
I am using avast! Endpoint Protection Plus.
I hadn't thought about attempting to ping to the server when this happens. That is definitely worth a try to see if it can even establish a connection to it at all. The issue I am referring to in the original post deals with trying to access it through the run command ( \\servername ) and attempting to access it through the Network section on Windows Explorer. The run command could not find the server and the server does not pop up in Windows Explorer.
I'm not sure how to go about checking to see if SMB1 traffic is blocked when the network is down. However, in my research of SMB1 connections, I did find that software such as WannaCry use this type of connection to spread Ransomware. Yikes. You may be right about migrating off of the Windows 2003 server. This makes me curious, though. It could be that Avast sees the SMB1 connection and is blocking it as a precautionary measure to prevent anything malicious coming down the pipe.
This is definitely something I had not thought of before, nor would I have thought of til you mentioned it.
As for the DNS server, we do use a local DNS server. I'm not entirely sure as of right now, but I think it is hosted on the Windows 2003 Server, which, as you suggested, should probably be moved over at some point.