Session: Error while trying to free lock for ci_session

I'm not an experienced with managing Redis and/or Memcache servers, but yes - being cache stores, when they run out of memory, they'll just drop the oldest entries, so that's a possible cause.

With Memcached, this is unavoidable as it doesn't support any kind of persitance.
Redis can persist data, but doesn't by default (dunno how the appropriate settings are named).

In fact, I just stumbled upon this earlier today: https://medium.com/@adil/storing-session....6r43twd8x