Hello from Germany,
i think i did a wrong step while doing this challange.
I am hanging at the point where you have to create a look up relationship for activities to properties.....in first place i've done that but nothing appeard on the app. Then i deleted the look up to do the same step again. Now i get an error message:
"Validation Errors While Saving Record(s)
There were custom validation error(s) encountered while saving the affected record(s). The first validation error encountered was "Lookups on activites must have a unique domain. You can not have multiple relationships to the same object."."

Now i can't continue with this challenge.
 

I'm doing the simple lightning components challenge and have hit this problem in my existing trailhead org and a brand new dev org that I've just created:

Challenge not yet complete... here's what's wrong: 
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: QVWBQHAG