I keep intermittently running into this error while working through different Trailhead exercises. Sometimes it disappears without my changing anything in the org at all (except letting time pass and possibly logging in/out of open instances of different orgs - both playgrounds and business orgs).
This time, it doesn't seem to want to go away on its own - but I'm able to successfully verify all steps in the Trailhead challenge both before AND AFTER it.
Has anyone else been running into this error, and if so, how did you resolve it?
(Note: I would be attempted to assume the error message is correct in its suggested resolution and I simply need to create a new org to get around the problem, but the frequency with which I've been running into this issue and its been self resolving leads me to believe this is more of a catch-all error they throw, and that my real issue might just be related to caching or some small change I can make, and maybe there's a simpler trick to solve it than having to recreate all the work I just did in another org.)
"Step not yet complete in My Trailhead Playground 1 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: XEITCEJL"