People "sometimes" get this error message when accessing the update-site via Eclipse or Tycho.
Using P2-Brwoser, I could track the issue down to p2 downloading
http://www.xpect-tests.org/updatesite/nightly/content.jar, which doesn't exist. Instead gh-pages sends an HTML-Webpage elaborately announcing the HTTP 404. P2, on the other hand doesn't recognize that a HTTP 404 happened, treats the HTML-contents as a ZIP-file (assuming it's a contents.jar) and fails.
People "sometimes" get this error message when accessing the update-site via Eclipse or Tycho.
Using P2-Brwoser, I could track the issue down to p2 downloading http://www.xpect-tests.org/updatesite/nightly/content.jar, which doesn't exist. Instead gh-pages sends an HTML-Webpage elaborately announcing the HTTP 404. P2, on the other hand doesn't recognize that a HTTP 404 happened, treats the HTML-contents as a ZIP-file (assuming it's a contents.jar) and fails.
As a workaround, you may use the update-sites http://www.xpect-tests.org/updatesite/nightly/local/ and http://meysholdt.github.com/eclipse_jdt_junit_runners/updatesite/
The "original" update site http://www.xpect-tests.org/updatesite/nightly/ is just a composite update site delegating to both of them.