The current update site references release 6.8.0 which is broken.
E.g. this file cannot be downloaded:
http://sourceforge.net/projects/eclipse-cs/files/updatesite/6.8.0/site.xml/download
By the way, it would be helpful to provide update site for older releases, for unexpected situations like this one or incompatible changes.
Please read here for more information
https://sourceforge.net/p/eclipse-cs/discussion/274377/thread/746a8014/?limit=25#99a1
For the time being update site functionality is restored.
Older releases of the plugin are available as update site archives in the projects download files section:
https://sourceforge.net/projects/eclipse-cs/files/Eclipse%20Checkstyle%20Plug-in/
Unfortunately multiple update sites won't guard against major project hosting outages, and maintaining them would be a PITA for me as a mostly one-man, spare time project.
All systems nominal