@rmolina, Just a note on the Section to add the property.
It is Ambari > HDFS > Configurations >Custom core-site > Add Property
I believe, Adding properties under Advanced Core site is not possible.
Just want to add to this post, there is actually lots of ways to fix this issue, but the root cause of the issue on Sandbox is that setting "yarn.acl.enable" to true. It should be set to false since, which is what Ambari does by default for non secure clusters. The Sandbox team is looking into this and probably fix in the next update sometime in January. Bug reference is BUG-48765
what is dr.who ? Can some one explain me
I saw in my environment for yarn.admin.acl - yarn, dr.who
Dr.who is the default username/static for hadoop core property : "hadoop.htttp.staticuser.user"
The description say's : The user name to filter as, on static web filters while rendering content. An example use is the HDFS web UI (user to be used for browsing files) See this:  http://hadoop.apache.org/docs/r2.8.0/hadoop-project-dist/hadoop-common/core-default.xml
Hope this answered your question.