Nether: Chunks suddenly disapearing after some time

Reproduce:
When entering the Nether, walking straight forward for eg 5min (Because of within Nether walking straight is difficult, I reproduced this above the Nether ceil)suddenly Chunks in all directions disapearing. Only some in the nearest distance stay.
When walk to the Chunk border, the next 1 or 2 Chunks get loaded - moving your head to left or right let them disapear again.
Jumping over the chunk border let you fall into void - endless. In my test I fall to y = -500 without dying. Then the position gets resetted to the coordinate after jumping - and falling again.
On "Minecraft game output" I found this entry at the time, when chunks disapearing: "Can't keep up! Is the server overloaded? Running 306211ms or 6124 ticks behind" (but it is not a Server game, its local! The CPU isn't overloaded (its about 20%))

 

I have a video of this, but its over 300 MB. I can't append it here, but you can contact me, if you want.