[gaunab]

Is there a more-or-less similar thing in Open Source? I'm looking for something like this for quite some time, but would need it to be self-hosted, and what would be even better, end-to-end encrypted (e.g. with shared secret).

[TuringTest]

If you just care about functionality, Leo Editor offers similar features, and adds in being scriptable in Python.

If you also care about usability and zero learning curve, I'm afraid I haven't seen anything similar which is open source, after being searching it for years. The closest thing might be one of several content managers/social networks a la MediaGoblin or Diaspora, or something more flexible you'll need to build your own workflow on some open wiki platform like Twiki or Zwiki.