After I killed the EnderDragon the EndPortal got activated. As I jumped in to it the client display turned black (see attachment). But Sound working well. After teleporting the player back to the End island, the end dialog started.
rejoin does not fix the problem. server is vanilla. server restart does not fix the problem.
Client log > "endportal_fail.txt"
Is this still an issue in the most recent versions (currently that is 1.10.2, or 16w42a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time.