I tried to create a brand new playground and redo the first and second steps, but after I have fixed some errors on the reports, it fails with this message

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: QRCAJQJE

I've got this error checking the step 3

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: EMFXYXCZ

It is a new Trailhead playground

Thanks, 
Vincenzo.

 

Since the points on some challenges are changed from 100 to 500, the related trail ( that before was completed ) appear in the list of those in progress ( from the dashboard ). I tried to take all the new challenges but doesn't log the module as completed, and consequently the trail. I knew that if you take the challenge again you don't gain the points, but I thought that at least it would mark the trail as completed.
 

I've got this error checking the step 3

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: EMFXYXCZ

It is a new Trailhead playground

Thanks, 
Vincenzo.