Strange, it happens all the time on master and i see there's the same problem on One also. I don't think it's easy reproduceable, it mostly happens when the player takes alot of damage being it from a boss or from another player. It seems like the player dies and miliseconds after he is resurected visually with 1 hp and he can't move but he still can cast healing spells, the damage spells can be casted too but they are resisted by the targets. So basically the player is stuck in this "zombie" state until he relogs, and crashing the server in the process. GM commands don't work either, .revive because he isn't actually dead, and .die because he isn't actually alive.
So as i said, happens randomly when the players is affected by big amounts of damage from different spells.