Loading terrain... (1.16.5)

Hi,

the debug client told "loading terrain... " for ever. The only console output was "Can't keep up! Is server overloaded?" etc. I tried to restart my client. After rejoining almost everything worked.

There's really nothing hard going on: The only biome Forge is allowed to spawn is a custom one with two colors of wool (height variation 0) and one tree and one piece of grass per chunk. In the world I loaded however the world wasn't flat. There were no trees at all and the grass was also missing.