Because it just recently happened, I believe we had a small map crash.
Anyways
If you get it when you enter in the game (Clicking start from your character menu), setting view distance to zero, and turning off water reflection helps with that.
If you installed any particles, check if their are any marked _0 or if you have any duplicates.
Meshes and items may have been placed in the wrong files. Check those.
If it is something different than this, I may also have that solution, but this is all I can think of right now. xD