In November 2015 I completed SalesForce Trailhead Developer Beginner. Now, during Christmas time I was able to complete the main challenges of the Admin beginner. And finally, during January I completed it.
To me this course was easier than the one related to development: so much configuration activities and the challenges are easier.
The big challenge to complete the training was one of the challenges. The checker was not working fine. After opening a question on the trailhead forum, I was contacted by a developer who was checking my development environment. After some checks, he noticed that the there were some elements that I could not see anymore, and by that reason the challenge checker was not working. I created a new DE Org, I completed the challenge from scratch and it worked.