Hi,
it looks like if you are trying to contact a namenode in stand-by mode. Maybe you have a cluster on HA configuration? in that case the cluster has two namenodes (an active one and another one in stand-by mode).
Be sure that the namenode DSN is the active one in the request
http://<namenode>:50070/webhdfs/v1...
To check if a namenode is active, you can check the list of namenode and their status in CDH manager under HDFS istances or, if you know the namenode's DNS, reading the information on webpage
http://<namenode>:50070
You should have a sentence like
Overview '<namenode>:8020' (active)
for the active one.
Best regards