DaemonUmbra

and it happens with no mods installed?

According to Optifine's changelog, the version of Optifine you're using is not compatible with Forge, do you get the issue without Optifine installed?

Yes

This is detailed in the EAQ and in the logs section of my signature below

Please provide your debug.log in the manner described in the EAQ and the logs section of my signature below

What version of Minecraft? What version of Forge?

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

There are tons of mods that run on 1.15.2, don't let 1.7.10 and 1.8.9 fanboys tell you otherwise, just check CurseForge

Just to confirm, you're using 1.12.2, right?

That is also 1.12.2 and looks to be a vanilla jar

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

Forge doesn't load mods from version-named folders anymore, as Mojang's launcher allows you to create profiles (They call them installations, which I think is a bad descriptor of what they actually do) that allow you to select a Minecraft version and a Game Directory. This Game Directory is the folder that your mods folder, config folder, logs folder, etc. are placed in.

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

Don't extract it, if WinRar extracts it on double click then that's your problem, WinRar is naughty and has no business taking over the jar file extension. Run this once and try again. https://johann.loefflmann.net/en/software/jarfix/index.html

Can you specify a Minecraft version and a Forge version? (Please don't use words like "latest")

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

Please keep this forum in English. Please provide your debug.log as described in the EAQ thread and the logs section of my signature below

It's a library that Forge uses, and it's not hacks in the way you're thinking. In this case "hack" means "code that we have no choice but to use even though it's terrible and ugly and should die in a fire" https://github.com/cpw/grossjava9classpathhacks/

If you have Java installed correctly you can just double click the jar to run it.

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.

Please keep this forum in English. Please provide your installer log using one of the paste sites listed in the logs section of my signature below

Please provide a full debug.log as described in the EAQ and the logs section of my signature below

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

Please don't hijack other people's threads.