To reproduce:

• Log in as an admin
• Create a new Menu profile field with the following details:
  • Shortname: favouritecolour
  • Name: What is your favourite colour?
  • Required: Yes
  • Display on signup page: Yes
  • Options: Undecided, Red, Green, Blue, Yellow, Orange, Pink, Purple, Magenta, Cyan, Turquoise, Brown
  • Default: Undecided
• Browse to a course
• Add a forum activity called what ever you want and add the following condition:
  • User field: What is your favourite colour?
  • doesn't contain
  • Undecided
• Save the forum and log out.
• Log in as a student of the course.
• Browse to the course.
• Confirm: The activity is shown as available.
• Go to the page to edit your profile
• Click the save button without making any changes
• Browse back to the course.
• Confirm: The activity is now shown as greyed out.