As I have noticed, this is a Mac-specific bug. Following the workaround suggested for a known Tk bug on Mac (see link), changing the line which binds the menu method to the keystroke to the following:
self.bind_all("<Command-f>", lambda event: self.after(100,self.menu_file))
is "fixing" the bug. They also suggest to increase 100 to 150 on slower systems. Hmm..