icbaccountsd: Excessive number of unncessary syncs - Denying request for sync!

Update:

After a lot of troubleshooting, I have found that this is related to iCloud and, in particular, to my iPhone. I am restoring the iPhone and setting it up as a new iPhone. I have an iPad connected to the same iCloud account with no issues. Let's hope this will finish with the icbaccountsd issue.

My Mail got more responsive after deleting a lot of files (including the Envelope Index, ExternalUpdates and MessageUidsAlreadyDownloaded) and deleting and fixing a few abnormal accounts that were present in some applications (Mail, Notes) but didn't show up in the Internet Accounts section of System Preferences. It seems the migration from previous OS left some "garbage" in my system.

Another update:

Once I connected the restored iPhone (clean, no apps) to iCloud, my Mac was logged out of the iCloud account (no Signed Out). The password was missing, some of the services were active (Safari, Find my Mac) and some were inactive (Mail, Notes, AddressBook) but I could not change anything, as the Pref pane was asking for the password (email, contacts, and notes gone). In the Console, I read something about Keychain. Instead of adding the password again, that I've done before with multiple issues including multiple copies of Contacts in AddressBook, I decided to Sign Out from iCloud. Reviewed Keychain, but everything seemed OK (no problems after verification). I found multiple Apple ID entries and deleted all of them (all these with some restarts... this looks more and more like Windows). Signed in in iCloud again.... and since then everything fine. No more icbaccountsd messages. Now, reinstalling all apps in my iPhone (no fun at all).

Good luck,

?Last update:

I was braved enough to activate iCloud keychain (I had all my passwords in iCloud and none in my mac) and iCloud went crazy. I was logged out again of Mail, Notes, and Contacts, the Console was flooded with icbaccountsd messages (problems with mDNSresponder, dnssd, accounts not found,...). I decided to unselect iCloud Keychain and keep my passwords Local. I don't really need iCloud Keychain (it is a good idea, but it seems poorly implemented). Something with my Keychain makes the system crazy (passwords from the first version of Keychain), that the Keychain First Aid cannot detect, but make the iCloud Keychain crazy.

Maybe all my problems were caused by iCloud Keychain, and unselecting the box in iCloud was the answer. I hope that my experience can help someone else.

Good luck,