The release 2.3.1 download link from the desktop client page goes to https://download.owncloud.com/desktop/testing/owncloudclient-2.3.1.tar.xz, which doesn't exist. All I see there are release candidates.
Please report such dead links directly to https://github.com/owncloud/owncloud.org/issues
Thanks, PR incoming.. https://github.com/owncloud/owncloud.org/pull/1186
If you want to be on top of development, you can also checkout the 2.3 branch from https://github.com/owncloud/client