Hit this during my attempted upgrade to Lucene 5.1.0. (Yeah, I know 5.2.0 is out, and we'll be using that in production anyway, but the merge takes time.)
Various tests of ours test Directory stuff against methods which the security policy won't allow tests to write to. Changes in FSDirectory mean that it now demands write access to the directory. 4.10.4 permitted read-only access.