Build error in Antlr tasks

I got back from work last week and my project using Antlr 4 for C # was building fine. I am getting started this morning to resume the project and I am getting this build error:

Antlr4.targets (129.5): Error AC1000: Unknown build error: Object reference not set to object instance.

I haven't made any changes to the code since I had a working build last week. This is a problem that affects my entire team. I tend to think this is an environmental issue because I can take the same solution and copy it to a laptop that has been outside of our domain since last week and it can build the project just fine. At this point, I can only guess that our IT system clicked something that changed the configurations, but I don't understand what that might be, so I can fix it.

Any help would be greatly appreciated. Thank.