My network has one computer (A) connected via ethernet cable to a router. There are two other computers on the network (B and S), both of which are connected via WiFi. On S, there is both SQL Server Express 2012 and 2017, hence the S for server.
I have a program that is found on both A and B. The first thing it does when it starts up is attempt to connect to the 2017 SQL Server on S. If that fails, it waits a little while to see whether any other computer will send it credentials. If it gets credentials, then it attempts to use them to connect. If it isn't able to connect in time, then a form pops up showing all SQL Server instances visible on the network, and allows the user to select one, add credentials, test connections, and so forth.
Also, both A and B have SQL Server Management Studio (SSMS) installed.
Up until last week, all was well. A and B could connect via the application or via SSMS. Yesterday, B could connect, but A cannot. B can connect using either the application or SSMS. A can't connect by either one. The application is getting credentials from B and trying them. I can see that happening, but the connection attempts fail. This eventually leads to the form showing up, at which point I can see both SQL Server instances on S.
I have also tried telnet from A to S on port 1433, and it works. So, A can more or less see the SQL Server, and the SQL Server is responding, since B has no issues, but A can't connect.
I've tried rebooting...well, by now, everything including the router, and done so a couple times over for most parts. A will not connect. B always connects. The last time the system was used, which was last week, all parts were working as they should.
I'm at a loss as to what to look at next.