Yes, the error message is a .NET framework error message. The error message is not generated by the UCCNC software, but the .NET framework. It is probably an error in the screenset file.
Unfortunately if you could not make the UCCNC modifications to work then there is very little to no chance that you will have luck with Mach4 and it's scripting language (Lua), because it is much more complicated.
But again, good luck to you, hopefully I will be wrong.