The registering device does not have appropriate credentials

I'm getting the same thing. I wonder how wide spread it is.

I'm getting this also. I didn't see this post before I posed mine: https://discussions.apple.com/thread/5206792 other wise I would have just addded to this post. It's been 3 days for me but only on one machine.

I logged out of iCloud on my Mac Pro then logged back in. I followed the instructions basically from this post -

https://discussions.apple.com/thread/5205557

Me, too. Just in the past couple of days on certain machines. Log out/in to iCloud does not work. Removing Flash data (?) does not work. iMessages does not work.

Correction: In System Preferences -> iCloud: Sign Out/Sign In, in System Preferences -> Flash Player -> Advanced: Delete all local storage... then restart the computer. Now able to sign into iMessage. iMessage works again.

Thanks Larry. That fixed my ability to log into iMessage again. I had to go back to my iPhone and re-enter the account info there too. I then closed out of iMessage on my Macbook Pro, waited about 10 minutes, and then my account addresses and phone number from my iPhone setting sync'd back to my Macbook Pro. Everything appears to be good now.

Yup this helped! Annoying but it's definitely the solution!

Just changing the Flash Player preferences fixed the problem for me...

in System Preferences/Flash Player/Advanced, delete all local storage, then restart the computer.