The CheckboxWidget object was in conflict with a php extension we have installed called PDFNet. (PHP wrappers for PDFTron software.) There were a few objects with common names that were in conflict with different extensions/packages, I didn't check them all. Examples: Page, Widget, Filter
In order to solve this problem, I went through all the code and renamed the objects. This issue could easily be avoided by using namespaces and I'm surprised that the code doesn't support that.