Hello,
That JSON is a rate-limit message. In short, the Xbox sign-in service has blocked your MCC login because there were too many attempts in a short time. The blank dark blue screen is the same problem. The fix is to clear the Store and Xbox sign-in state, then let Windows create one fresh session that MCC reuses, and avoid repeated prompts.
Close MCC completely. Sign out of the Xbox app and the Microsoft Store. Press Win key, type WSReset, then run it and wait for the Microsoft Store to reopen. Now give the rate limit time to cool down. Wait at least 10 minutes with the game closed and no sign-in attempts.
After that, open the Xbox app first and sign in with your Microsoft account. Open the Microsoft Store and make sure it shows the same account. Finally start MCC and when it asks to sign in, accept the account it offers and let it finish. Do not re-enter your info multiple times if the window takes a moment to load.
If it still loops after you follow those steps exactly, tell me whether you are on Steam or the Microsoft Store version and which Windows version you use. I will give you the next fix right away.