I also experienced the problem... But in my situation, my friend just step onto a piston(not sticky) when it is activated with no block on it at the same time. Though the background music still playing, everything else related to the game freezes.
By the way, when the game freezes, my friend disappeared in my sight.
Sometimes activating a lot of pistons at the same time also crashes the client(server works fine). Maybe the crash has something to do with the ticking mechanism.
I also experienced the problem... But in my situation, my friend just step onto a piston(not sticky) when it is activated with no block on it at the same time. Though the background music still playing, everything else related to the game freezes.
By the way, when the game freezes, my friend disappeared in my sight.