If you are on a new device, you may be at the device limit. Try going to brave://rewards-internals and go to Event Logs. See if any thing says Device Limit Reached. If that is the issue you need to have an old device removed. If that is the case, you can solve that issue here: Device Reached Limit Cant Login - #6 by Herrvader
I had the same error issue several months ago and didn’t figure out what was happening until I looked at the logs.