Hi,
the router is rock solid when running, and I've configured failover on 11 devices around the world without out any issues. But my own test device ended with inconsistent configuration that could not be removed. Doing to many changes in the failover configuration can result in this.
Made a backup, reset the router using the reset button. Router fine. Restored the backup, and the issue is also restored.
Reset the router again, an reconfigured it from scratch. It's fine now.
Regards Peter