Working on the Lightning Experience Reports & Dashboards Specialist superbadge, I'm at Challenge #7 (Create the SolarBot dashboard) and getting the following error:

There was an unexpected error while verifying this challenge. Usually this is due to some pre-existing configuration or code in the challenge Org. We recommend using a new Developer Edition (DE) to check this challenge. If you're using a new DE and seeing this error, please post to the developer forums and reference error id: IWVYGERW

This is a new trailhead org, created excusively for completing this bage (no other Trailhead modules have been worked in this org). I had a similar unexpected error in a different org, on my previous superbadge -- also when creating dashboards, and corrected it by doing a dashboard refresh. Tried the same thing here, to no avail. Even with a freshly "refreshed" dashboard, I get the error. 

Any suggestions? Could not find the IWVYGERW error code in discussion forum searches.