I just tried to check out a fresh copy, which did not succeed.
The URI was a subpath of trunk, so not trunk itself.
Checking out a fresh workspace because C:\jenkins\workspace\ProjName\MyFolder doesn't exist
Cleaning local Directory MyFolder
Checking out svn://srv/repopath/trunk/MyFolder
ERROR: Failed to check out svn://srv/repopath/trunk/MyFolder
org.tmatesoft.svn.core.SVNException: svn: E155007: 'C:\jenkins\workspace\ProjName\MyFolder ' is not a working copy
Checking out to a 1.6 workspace instead of 1.7 worked with the same configuration.
Checking out trunk to a 1.7 workspace fails as well, same error.
Checking out a 1.7 working copy manually - to the target path - before the build starts (so, without jenkins),
jenkins does run an update on the working copy, the checked out path.
Going back to an older revision on the 1.7 working copy Jenkins does indeed update to HEAD.
Just the new checkout to 1.7 workspace seems to fail.