svn: E155021: This client is too old to work with the working copy at xxx (format '31')
I had this problem on Mac OS Yosemite with Webstorm and IntelliJ.
svn --version on the command line printed 1.8, but even with command line client option enabled with value svn in Webstorm/IntelliJ I was getting the warning.
Running which svn and pasting the value (/usr/local/bin/svn for me) into WebStorm->Preferences->Version Control->Subversion->Use command line client removed the warning and fixed the issues I was having in Webstorm and IntelliJ.
The working copy has format understandable by SVN 1.8 client, but your client is SVN 1.7 or older.
You should double-check what Subclipse / SVNKit / svn.exe client versions you have on your system.
Try accessing the working copy from command-line and see whether you get the same error.
If you definitely have svn installed and running at the correct version you want, this error means the metadata in your project refers to an older version of subversion. Upgrade it by following this guide: http://www.robsearles.com/2008/12/svn-client-is-too-old-to-work-with-working-copy-solution-using-rsync/