The likely cause of this is the email being invalid and marked as Bad/Invalid. Once you unmark the email as bad or update as a valid email, you can save the Custom Field successfully. To edit the Custom Field and Users Bad Email Indicator, navigate to:
  1. Core > People Finder
  2. Search the User
  3. Click Edit in Internet
  4. Unmark the Email as Bad or update the email address
  5. Click Save
  6. Click Edit into General Information 
  7. Enter in the Custom Fields as neeed
  8. Click Save