Get the attached crash when trying to save an area selected ingame in 1.16.2. On a multiplayer server running paper.

Only files I have in mod folder are:

• fabric-api-0.19.0+build.398-1.16.jar
• litematica-fabric-1.16.2-0.0.0-dev.20200824.031346.jar
• malilib-fabric-1.16.2-0.10.0-dev.21+arne.1.jar

crash-2020-09-08_10.55.16-client.txt

Looks like you are trying to save a massive area? How big is the selection exactly?
To not get the crash you should shrink the selection volume as much as possible (for example vertically, don't save empty space up to y = 255), and in general it would be better to use multiple smaller sub-regions instead wherever that makes sense to tightly capture only actual existing blocks/structures.

Litematica and schematics in general are also not ideal for world download type usage, as the selection is all saved into one file and basically one sub-region becomes one array of blocks. So if you select areas multiple kilometers in dimensions, the number of entries in that backing data array at some point overflows the maximum (max positive integer, ~2 billion entries, and how many block positions that is exactly depends on the number of different block states, but it would usually be probably around 10-12 billion blocks in volume). And since all of that data needs to be loaded to memory at once it will also be extremely performance intensive handling such massive schematics.

So if this is your use case, then a dedicated world download mod is a much better choice, as it saves the world like the game does normally, using chunks and region files where only the area around the player is loaded at once, and the world is loaded and saved and handled in smaller pieces, ie. Chunks.