So today I started with my MBP setup with iCloud account "B" using all iCloud functions including Keychain, iPad Mini using he same iCloud account, functions and Keychain and my iPhone4S using iCloud but not ever having used the keychain. MBP running 10.9.1, ios7.0.4 on both devices. I come to add my iMac (also on 10.9.1), to the iCloud account and Keychain. Going through the setup it gets to adding the keychain and I choose the option to verify via another device active for Keychain. First odd thing, the request to verify does NOT appear on either the MBP or the iPad Mini. From this point neither iOS device can: sync Exchange mail (imap mail account is fine), Exchange Calendar, or load Safari. All these symptoms have an iCloud-lockout/access issue flavour to them. The MBP continues to have normal Exchange access via Outlook 2011, as does the iMac, bothe have normal Safari function. All attempts to get around this fail. The Keychain even starts resetting as soon as enabled leaving me at one stage with the iMac request visible on the MBP but Keychain unchecked on the MBP causing an infinite loop. Only way out is to force quite System Preferences (which makes me VERY nervous). Logging out of iCloud on the MBP results in NO devices having Keychain access enabled (the ios7 iPad got kicked off at the first Keychain reset and would not re-enable). Multiple Keychain activations via Verfication code by SMS to my iPhone (to try and re-enable the MBP), are received at the iPhone but fail to break the lockout type condition. Apple Support are engaged (5min callback - excellent!). Past 1st level to 2nd level, not a known issue, no problem evident from Apple supports point of view with the iCloud account, number of devices etc etc. Even, under Apple's direction, trashing the iCloud folder under Library and restarting does not recover, the Keychain enablement attempt that can never complete comes back after the restart. (cue spooky music). Now the killer, on BOTH iOS devices the iCloud account B cannot be removed, Find My iPhone etc is removed ok but the "saving" never completes (again looks like iCloud locking them out). After much testing the ONLY recovery is to....full software restore and restore backup to a stage with no iCloud account configured. Now I use the phrase "iCloud account B" because I had this same issue on my previous iCloud account over a week ago, in that case I was left with having to sign out of the iCloud account A on all devices, restore SW and restore from backup both iOS devices AND create a new iCloud account "B". I put it (iCloud account A), down to: One of those things, random corruption etc or Possibly an app that was replicating iDisk functionality therefore writing to the iCloud account - so the app was deleted from both iMac and MBP and has never been involved with the iCloud account B that failed today. Apple support are calling back tomorrow when we intend to allow them to setup an iPad with iCloud account "B"'s details to see if they can replicate and take a dump to progress diagnostics (I'll trust them to delete it afterwards or I can remote wipe it if needs be). Horrible, nasty bug/interaction that now, I can only put down to iCloud function - neither the iMac nor MBP exhibit any other issue whatsoever, both are updated on OSX and apps. Similarly both iOS devices are at 7.0.4 and have had pristine iOS lives with no other problems to date. Thoughts anyone?