DaemonUmbra

Please provide your debug.log using one of the paste sites in my signature below

Because you might as well just use new BlockItem directly and not auto-assign the item to Building Blocks

Please don't double post, I've merged your threads into one. That has nothing to do with a crash, please provide your debug.log files from both server and client using one of the paste sites in my signature.

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support.

Is that running it from a console window or are you still double clicking the jar?

I'd bet if you would post the log where you say "EVEERYTHING CRASHES" with Java 8 we could discover either: Your PC isn't actually using Java 8, or The crash has nothing to do at all with Java version

Use 1.14.4 or 1.15.2

I don't have the knowledge or experience to tell you

If you installed Java 8, make sure that's the one your launcher is using to run Minecraft.

Your best weapon against cheaters will be server staff with moderator/admin tools.

This means you have a 32-Bit JDK, you should uninstall it and get a 64-Bit JDK

Use JDK 8 https://adoptopenjdk.net

What do you mean "now"? Cheat clients have always had the ability to lie to the server. Also, just my opinion, but you saying that Minecraft is "showing it's limits finally" makes me feel like you're looking for something to look down on Minecraft for. Edit: All that is needed to defeat that mod is a mod that pretends to be it that responds with a packet that's filtered to not mention the illegal mods that are installed.

Run the jar from a console window, what does it say?

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support.

Do you see the "New" button there?

Yeah, that installation called "forge"? That's made by the Forge installer and gets replaced every time you install a different version of Forge... THAT's what I mean by pre-made

Honestly you should always make a custom installation for modpacks, don't use pre-made installations because they tend to be subject to change

Fairly certain you're confusing "uninstall" and "deleting the installation(really a profile but Mojang calls them installations) that uses 1.14"

Unless the author gives permission (which I doubt he will, seeing as he moved his mod over to his own game), nobody can legally port it to a new version. Also, 1.12.2 is no longer supported here.

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support.

You're not running other games at the same time, are you?

Please don't hijack other people's threads if you have an issue make your own thread with logs

Try a more recent version of JEI

This can be done without events, just override onBlockActivated in your block