The default version for the owncloud client on Linux Mint (17) is completely outdated; it will not properly connect to current owncloud servers version 7 or 8. Here is a quick install for the latest version on Linux Mint 17:
sudo apt-key add - < Release.key
sudo sh -c "echo 'deb https://download.owncloud.org/download/repositories/8.2/Ubuntu_14.04/ /' >> /etc/apt/sources.list.d/owncloud-client.list"
sudo apt-get update
sudo apt-get install owncloud-client
Update 2017-09-05: The repo can no longer be found at download.opensuse.org. owncloud.org now hosts this repo.4 People think it's helpful 😀