This error is caused by not having the latest version of git due to updates in the server-side application. I recommend trying to access it through a different protocol (git:// or ftp) if possible. Otherwise, just check for updates in the egit plugin.
Update:
I have learned for a fact that github supports the git protocol. You can use
git://github.com/SpringSource/spring-mvc-showcase.git
as the repository. (If egit asks for a protocol separately from the URL, change it from https to git).