I hope this helps you… But for me, it was removing the Rocket Chat instance from their end https://cloud.rocket.chat/ and re-register/resync from scratch.
I had to find the : more button on the right and select Remove/Delete workspace (which is hard to find!).
After that’s cleared out, I tried adding my instance to their https://cloud.rocket.chat/ and resync. Seemed to come back OK.
Good luck!