After chatting with fuzzie, bdonlan, nornagon, and diagonalfish, it seems there's no longer any way to access this repository.

I've created a new GitHub Organization and repository at openc2e/openc2e for new development.

At the time of writing, changes include:

• Making it build on modern systems, with SDL2, Qt5, Python instead of Perl, and no more Boost
• Simple builds on Windows (just open in Visual Studio!)
• New dev tools (praybuilder, etc) built on file format code
• Other updates

If you're interested in contributing or running openc2e, please join me there!