Qt5 with msdev2017 - Changing additional include directories breaks CL compiler

[RBLL]

I have a very odd problem. I started a new project using QT5 with the Visual Studio 2017 plugin. Everything was working fine: building and executing without any issue until I needed to add some external dependencies. I modified the project properties "additional include directories" to add a couple of folders. Once added, the program would not compile anymore, giving me the following error:

Project ERROR: Cannot run compiler 'cl'. Output:
Maybe you forgot to setup the environment?

What is even more odd is that even restoring the "additional include directories" to its original value doesn't fix the issue.

Out of curiosity, I made a dummy project (QT5 GUI application). As expected, it builds and runs fine, but the moment I touch the "additional include directories" the CL compiler seems to break and there is nothing I can do from this point.

Any ideas what could cause that?

Thank you very much!