iCloud password prompt on OS X Yosemite startup Whenever my MacBook Air starts up from shutdown (or on restart), once the Finder has started, it prompts me for my iCloud password, like this:



As you can see there is no title or anything to identify the app or process requesting the password. However, through the use of a clever Python script (unfortunately I can't remember where I got it from) I was able to determine the process requesting the password:

UserNotificationCenter (/System/Library/CoreServices/UserNotificationCenter.app)

If I click Cancel twice (it pops up again after the 1st Cancel) it doesn't reappear and there are no apparent issues. All my iCloud services (Photos, Music, Contacts, Calendars, Keychain) seem to sync and work just fine.

I used Activity Monitor to dump the processes open files and the only file of relevance I can see is /usr/share/icu/icudt53l.dat although I have no idea what it is.

The problem only started happening about a month ago, and there's no obvious change that occurred on my machine that I can think of that might have caused this.

EDIT: obviously I tried entering my iCloud password, without success.
If you have another Apple device, such as an iPad or iPhone, and you deleted iCloud off of it, it can disrupt every device you have, and you will just need to log in again, possibly generating App-Specific Passwords for your Mac. This happened to me around the same time, but I can't remember if it was the result of the upgrade to OS X 10.10.4 that broke the connection of it was when I deleted iCloud from my iPad (I was lending it to someone).

You may need to log iCloud in with your Apple ID password, or if that does not work, log in to your Apple ID and generate an App-Specific Password for iCloud. 

The same problem cropped up for iMessage/Facetime as well. There you definitely need to generate an App-Specific Password, however I think I resolved the issue with iCloud by entering my Apple ID password. Once it sets in your Keychain, you shouldn't see the pop up again.