vb.net application stopped working on 64bit machines

I have a vb.net Windows Forms application compiled in Visual Studio 2013 using .NET 4.0 (not client profile), x86 target cpu.  The application has been running on multiple machines across dozens of networks for several years.  Suddenly a couple of weeks ago it stopped running on 64 bit machines on 1 network. 32 bit machines still run it from the same server. 64 bit machines are running .NET 4.6. Windows updates were rolled back, firewalls turned off, running as administrator, anti virus turned off, user network permissions are good. Any tips would be greatly appreciated.