The 404, System.NullReferenceException and related issues are fixed in 3.5.0 that was released yesterday.
I didn’t see why the socket was disconnected in the first place. There may be additional information in the server logs (memcached.log is the first place to look, also in the error logs - correlate the time with that shown in the exception).
And I don’t know the reason for the unambiguous timeout (unambiguous just means that it is guaranteed that nothing was updated by the request vs ambiguous - which means that maybe something got updated by the request. It’s not an updating request, so nothing got updated). Since it says the request was cancelled - to me that implies that it was sent (but I might be wrong), which would mean it had established a connection, but there was not response within the 2.5 second timeout. A longer timeout might help.