Having trouble rejoining a Matrix room after being banned and unbanned?

Sometimes it happens that your Matrix user account is banned from a room / channel in the Matrix network, but at a later time the ban is revoked.

You try to rejoin the room in question, but your Matrix chat application tells you Cannot join user who was banned, or something to that effect.  Invites from others in the room don't help you get into the room again — you get the same error message.

This is a little-known Matrix federation edge case (as of the time of this writing).

Here is a workaround I have tested works, courtesy of @flawedworld from the GrapheneOS Matrix network.

1. Make a tester throwaway account in your Matrix homeserver.
2. Log into your tester account using a Matrix client, like perhaps app.element.io in a throwaway browser window.
3. Join the troublesome room in question using your tester account.
4. Wait for the room to sync in your tester account's Matrix client.
5. Attempt to rejoin the troublesome room using your regular Matrix account.

Between steps 4 and 5, your homeserver will begin receiving events from the room in question again, and (this is just speculation on my part) clear up any stale information about your main account not being able to access the room.  This is (again, speculation) why rejoining in step #5 begins to function again.

Happy Matrixing!