In BucketingSink#createHadoopFileSystem one can find this piece of code:
FileSystem#get() wraps the created FileSystem in a SafetyNetWrapperFileSystem, resulting in the instanceof check to categorically fail.
We may want to replace the get() call with getUnguardedFileSystem(). We should also look for other occurrences of similar instanceof checks.
According to a thread on the mailing list this causes the BucketingSink to be unusable. http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/BucketingSink-broken-in-flink-1-4-0-td17710.html