Ronald Young

have been seeing the same thing in the last several snapshots.  Chunk updates drop off to 0, framerate skyrockets, and entities like animals freeze their movements, items cannot be picked up, and "Save and Quit" will just cause the game to hang, forcing it to be ended via Task Manager.

Obviously, the issue will need to be corrected, but a temporary workaround can be achieved by creating a datapack containing the contents of the "underwater_ruin" directory from the JAR under a directory just called "ruin".  This will allow the world to load and generate chunks correctly.

Personal experimentation is hinting to me that the crashes are in fact related to the missing structure files.  Here is why I say that:

18w20a and 18w20b client hangs on world generation in single player and will crash a server as well if one uses the seed WAITCHAIN and Large Biomes, giving comparable errors in the log about missing structure NBTs.  However, using a modified JAR file, I can generate and play in that world.  The fix?  Modify the client JAR file to contain a duplicate of the "underwater_ruin" directory just named "ruin".  I have not tested, but modifying the server jar in a similar way would, I suspect, solve the issue for a server as well.

Long story short, if the game can find the structure files, the crash does not occur.