iCloud sign out not possible on High Sierra I recently changed my Apple ID mail address. I was getting messages that I have to enter my password for my old iCloud address. Therefore I attempted to sign out on the system preferences. However every time I try this it gets stuck at some loading process. I have every iCloud service disabled. The internet-accounts panel says that this is not my primary iCloud account. I tried deleting the keychains folder and signed out and on again.
I was able to fix the issues by deleting the files in ~/Library/Accounts/ and ~/Library/Preferences/MobileMeAccounts.plist then logging out and in again before opening the panels again. Warning: This also removes every other account you have listed in the accounts folder.