Session: Error while trying to free lock for ci_session

Well, I don't have another response.
If I did, we'd have the whole thing figured out by now.

Is there anything else I can provide that would be useful to you as a core developer or are we on our own?

Best thing I could look at is the call details (method names & params, similar to a stack trace) for all CI_Session_redis_driver methods, in the order that they appear, for the request that triggers the issue (and ideally for the one preceeding it too). I'm afraid this is hard to obtain without monkey-patching the class to log that info.

I could also look at your code, the pieces that do session-related stuff anyway, but I highly doubt seeing anything related to this particular issue - that's a waste of time.

But in any case, that would (hopefully) give me just enough to make an educated guess ... I sense that you're relying too much on my directions, for something that you are more well-equipped for in this instance - I can't reproduce the issue, I don't even have Redis or Memcached, I can do literally nothing but guess. It's not that you're completely on your own, but rather you don't really need my help as much as you think you do.