Thanks for the reports on this. Our WoW and server teams are looking into these issues.
This error may clear itself up after 10 minutes usually, so try logging in to the affected character again after a bit of time. Try logging into a different character in the meantime if you have another one to try.
I decided to log into another character and log off properly for 30 minutes. After waiting, logging back in and trying the affected character I still receive the error.
I want to add to this that I even went to my character’s location and they aren’t physically in the world. Seems like they are stuck in some limbo zone. Guess they wanted to get to the Shadowlands early