>>The problem was resolved by starting the SQL Server Browse service on the server. What is strange, that before this service started, many PCs using the VFP app with SQL Server didn't have a problem. But then one PC (newer) had the problem. So I will make a note to always start the SQL Server Browse service.
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>This seems a little bit unorthodox.
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>First of all, if you have a firewall, all VM having a need to communicate with SQL Server will need to have their port 1433 opened towards SQL Server. That assumes SQL Server is also hosted on another VM. Thus, all Web servers and application servers, for example, would need to have TCP/UDP opened, for the inside interface, as we assume all would be denied by default.
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>On your end, it appears, either there was no definition or specific rules were added to support that as this was working for all servers except one.
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>There shouldn't be a need to start the service you mentioned, on the SQL Server itself, to workaround the issue. I have some doubt this is even related. Could there have been some other changes, meanwhile, that would have made that work at the same time?

It was within literally 10 minutes when the customer sent me email saying that they check everything, as far as firewall or AV and were still getting error. And then I started the SQL Server Browser service and asked him to try again. And he said "everything works now." It is, of course, possible that something else may have occurred at the same time. But I don't know.