DaemonUmbra

You do know what your mod can use the code of pretty much any properly labelled jar in the mods folder, right?

Dark Utilities appears to require Bookshelf and isn't indicating such in its mods.toml

You haven't done anything funny with file permissions, have you?

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

I've split your response into its own thread, now please provide your debug.log 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.

If you want to be modular then do it with jars instead of classes

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

Please provide launcher_log.txt from your .minecraft folder

Please provide your installer log using one of the paste sites listed in my signature.

Please don't hijack threads. Regardless, 1.12.2 is no longer supported here, please update to 1.14.4 or 1.15.2 to receive support.

If you must follow a tutorial, please follow McJTY's tutorial as it is the least terrible one currently.

Remove the bohem and tell the modder to learn Java and make their mod properly

Remove Vulcanite

Have you tried without Optifine?

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

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

You want mods to be universal, which is impossible with Forge. You want something like that, look at Sponge.

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

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

This sounds like you're trying to force users to use YOUR server and if they want to use a different server they first have to cancel connecting to your server. This sounds like a bad thing to do to users.

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

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

It looks like Minecolonies and/or Project Essentials have threading issues.

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