I tried to run the example workflow "HDFS_and_File_Handling_Example" contained in the example Server. Both operations upload/download work fine with the node "HDFS Connection". But when using the WebHDFS connection node I got the following error
Which port are you using for the WebHDFS connection node?
I would also ask you to make sure that you are using the fully qualified domain name (i.e. "someserver.company.com" instead of just "someserver").