GitHub has made changes to improve security, deprecating the use of git:// protocol. Our builds started to fail because of this, since the submodules were not being fetched. The solution is to update the submodule URLs to use https:// and the fix is already merged into the develop branch.
See Improving Git protocol security on GitHub | The GitHub Blog for more details.
Solution
All repository users should now update their forks to pull in the fix (see wiki for details) and then run git submodule sync afterwards to update the URLs in the local repository.