Indeed. Try cleaning the Parsers project and rebuilding.
I had already tried cleaning and rebuilding the entire solution, that usually took care of problems like that. This time, however, the issue persisted, that's why I posted it here.
When I tried your suggestion (cleaning and rebuilding only the Parsers project) I was able to build FO successfully. Interesting. Looks like I've to build the Parsers project separately from now on.