Unable to uncheck the "Encrypt Backup" box, in iTunes 11
I had a similar issue and documented the procedure here:


  I have found that this can be caused by installed Profiles
  (Crashlytics Provisioning Profile in my case, but some other people
  reported the same with an M1 (the telecommunication company) Profile
  installed).
  
  To delete it:
  
  
  Go to Settings > General > Profile, tap on the app’s configuration profile.
  Then tap Delete Profile. If asked, enter your device pass-code, then tap Delete.
  
  
  Then:
  
  
  Go to the Summary page for your device in iTunes
  Double check "Automatically Back Up" is set to "This computer"
  Double check "Encrypt iPhone backup" is unchecked
  Press "Back Up Now"