iMessage Authentication Loop on macOS Tahoe 26.5 Beta Issue Description: iMessage is stuck in a persistent login/authentication loop after updating to macOS Tahoe 26.5 Beta. The app repeatedly prompts for Apple ID and password. After entering credentials, it briefly attempts to sign in before returning to the login prompt or showing an "unknown error." This issue is isolated to the MacBook Air (M3); iMessage continues to work correctly on iPhone, iPad, and Mac mini. Related Forum Thread: Several other users are reporting identical symptoms here: https://discussions.apple.com/thread/256256620 Troubleshooting Steps Taken (All Failed): Standard Maintenance: Restarted Messages app, rebooted the MacBook Air, and verified Date/Time settings. Credential Management: Changed Apple ID password and attempted sign-in with new credentials. Manual Keychain Cleanup: Attempted to delete iMessage and com.apple.idms entries via Keychain Access UI (entries were persistent and could not be deleted). Terminal-Based Force Deletion: Executed the following commands to target specific registration tokens: security delete-generic-password -s "com.apple.facetime: registrationV1" security delete-generic-password -s "com.apple.imessage: registrationV1" Cache & Identity Purge: Cleared local authentication state and app caches: rm -rf ~/Library/IdentityServices/* rm -rf ~/Library/Caches/com.apple.Messages rm -rf ~/Library/Caches/com.apple.imessage Full Keychain Reset ("Nuclear Option"): Renamed the Keychains folder to force macOS to create a brand-new database: mv ~/Library/Keychains ~/Library/Keychains-old Followed by an immediate system restart. Current Status: Despite a completely fresh Keychain and cleared caches, the "unknown error" and login loop persist, suggesting a regression in the authentication subsystem of build 26.5.