imessage on mac keeps asking for password

I got the same problem with the repeating passwords.

I should mention that before stumbling on the iCloud keychain solution, I tried to create a new User ID on the Mac and assign it my iCloud ID. If I had to try again, I would disable the iCloud keychain on all devices, check to make sure that killed the prompts, and if so re-enable iCloud keychain. Looking back on it, I'm not so sure deleting login.keychain was really necessary. I needed to re-enter a few passwords for background apps, but so far so good. The login.keychain was recreated automatically. My solution was to disable iCloud keychain on all of those devices, then delete the login.keychain using Keychain Access, and restart. The problem seems to have been some corruption in the iCloud keychain, which I was using on my Mac, iPhone, and iPad. After trying numerous potential solutions, including nuking my login.keychain file, I think I finally have it figured out. If I entered my password, it would ask for it over and over and over again. If I cancelled the dialog, it would prompt a second time and then go away. It never said that it was for mail, messages, facetime, or anything other than iCloud. I was having a problem where my Mac prompted for the iCloud password multiple times every time I logged in. Any suggestions I could carry out myself? I would wipe my drive and re-install but (a) My Time Machine is in another country from me at present! and (b) I am nowhere near an Apple Genius Bar. Then a day or two later, one or other of these apps (it can be a different one each time) tells me I have entered it incorrectly again - and then it is refused on ALL of them - again! I then have to go through the same old procedure - and this is happening every day or two! I am becoming increasingly annoyed & frustrated! This only seems to be happening since upgrading to Yosemite, but that may not be relevant, of course. I then change it, carefully record the new Apple id, change the Apple id login on iCloud, iMessage, Facetime, App Store, & iTunes.

'you have entered your apple id incorrectly') when I have in fact entered the CORRECT Apple id & password.

I was having constantly recurring problems with my Apple id on my MacBook Air. I have been having a similar problem that I, perhaps wrongly, posted on the Yosemite forum.

Imessage on mac keeps asking for password