Server freeze - channel timeout

Situation: We have Photon standalone, on-premise server, serving PvP games. Each game is played by 2 peers and has its own PoolFiber. Each action from peer is directed to corresponding game using RequestReplyChannel. Under a load of some 700 simultaneous users and 350 games we experience a 'freeze' situation - e.g. request received from peer, goes to channel and then - timeout. Strange thing is that we are logging requests on game side, in channel subscriber function. That log is empty - as if subscriber was not called at all. How is it possible? Server has 24 cores, 16 GB RAM Thanks