I am not sure but i think you ran out of ram like he said. Download the world to your computer and launch it on your PC. Then just shrink the WorldBorder. Try using /worldborder set 100000 after, press Windows + R at the same time and type %appdata%\.minecraft\saves. then get your new save out of there and upload it as WorldFile. If you dont know what your save is called, just go to your minecraft and check the World Name. if there is multiple world called with same name, check the name below the world.