Hello, I got somes problems when I try to load some schematics ...

Unfortunately loading old schematics is currently not possible due to the "flattening" of blocks in vanilla 1.13. I still need to look into if it's possible for me to use the vanilla update code for converting those old schematics, but I suspect it won't work properly at least if the schematic has any modded blocks.

Currently the best way to bring schematics over, is to create an empty temporary (void) world in MC 1.12 (and maybe set the randomTickSpeed GameRule to 0 in it to avoid some possible updates) and paste the schematic into that world with MCEdit-Unified. Then open that world in 1.13.2, letting vanilla convert the blocks in the world, and then make a new Litematica schematic of the stuff from that world.

You might be able to use the datafixer stuff built into MC that is used for updating the worlds to update the schematics.

Specifically net.minecraft.datafixers.fixes.BlockStateFlattening

I have builds already on my server with mostly working old schematic import support for 1.13.2 and 1.14.4. I'm using some of the vanilla flattening data, but the vanilla data fixers aren't directly usable unfortunately.

Another idea instead of using old versions before the latest, so like what I read was using 1.12 and loading the schematics with MCEdit, you can always be in the current version, and use the WorldEdit mod to paste in your old schematic, I'm using 1.14.4 and I got the WorldEdit mod with Litematica and I noticed this issue so I found the folder where to put schematics into the mod, and in game in single player where you got access to view the schematics, I did //schematic list and I saw all the ones in there. You can now get your old schematic with that and do what you want with it.

@RacistTim you can now load and paste *.schematic files with the latest litematica from https://masa.dy.fi/tmp/minecraft/mods/client_mods/ in 1.13.2 and 1.14.x