When the binaries for SVN were updated to version 1.9, they may have not been built to support SASL encryption. This is inconsistent with the old binaries, which did support SASL.
Error svn: E170013: Unable to connect to a repository at URL 'svn://RootName/RepoName/trunk/ProjName'
Error svn: E210007: Cannot negotiate authentication mechanism
Error Failed to connect to source control. Check your settings and connection then try again.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-36829 in the post.