i have VisualSVN Server on the same machine and it was upgraded to latest version today.
I wanted to upgrade Redmine using SVN Update (as i always do) but got this even after restart
svn: E155036: Please see the 'svn upgrade' command
svn: E155036: The working copy at 'C:\redmine'
is too old (format 29) to work with client version '1.8.5 (r1542147)' (expects format 31). You need to upgrade the working copy first.