Edit by @Lunatrius: you can try to set the VBO option in the settings to OFF or remove Optifine while using Schematica. I am hoping that this will no longer be an issue in 1.13.

optifine version (from file name): OptiFine_1.12.2_HD_U_D1
setting that i found to be causing a problem with schematica: "Render Regions" in performance settings

if I set render regions to on, my schematic would only load in partially where it would just show outlines of blocks but not their textures and after a few seconds the game would crash and the launcher would display this error: "java.nio.BufferOverflowException".
i uninstalled all my mods and narrowed the problem down to a conflict between the newest version of optifine and schematica.

i was on a server at the time and would join the server (a spigot server with some multi server plugin), load in the schematic i was working on, move it to the correct coordinates and then after 10-20 seconds the game would crash. that schematic was the main part of "the world eater" designed by ilmango

crash report in pastebin: https://pastebin.com/WieGmqrZ

I am not willing to add support for Optifine because I do not have the time to reverse engineer the Optifine code to check what is actually going wrong with their "optimizations".

FYI This is bugged over on the Optifine issue tracker right now.

sp614x/optifine#1356