I had this same issue, but was eventually able to log back in.
Solution: I completely closed Diablo 4. Then on the task bar I exited Battle.net. Waited a minute and then restarted Battle.net. Logged back into Diablo 4, and was able to get in the game with no further issue.
Note: Some people have mentioned they had to restart battle.net several times before it worked. It only took me 1 restart.
Note2: Someone mentioned having to sign out of battle.net and sign back in to fix this.
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