OK it works. Client is implementing things quite differently so I finally rewrote server player motion algorithm
At this stage, the player is able to move by prediction between two
updates. The speed is just not the right one. It has to be computed
depending on the FPS of the client. The server is currently set for an
update every 3 seconds
OK There were still quite a number of cases not handled. It should be
completely done now :) . Still need to: do a lookAt() on each prediction update