After killing the ender dragon, when jumping into the End portal, the credits starts rolling but client gets timed out in about 10 seconds.

Then server continues running but gets crashed by Watchdog.

Disabling whatchdog doesn't change a thing, it just prevents the server from auto crashing.

When reconnecting, player gets placed near the end return portal. 

If I try to skip the end credits by pressing echap i end up beneath the end island as if i was falling into the void without taking damage except the island itself is invisible and endermen are not moving.

Setting gamemode into creative, flying and then going through the portal does the same except i can fly around the invisible island, and phase through endermen.