DaemonUmbra

First of all, why are you shadowing log4j? Minecraft already provides log4j2. Second if build is running shadowjar instead of jar then you might need to tweak the line where it says jar.finalizedBy('reobfJar') to finalize shadowJar instead

What do you think you should do about this?

What version of Minecraft are you using?

It's looking like something in the save is corrupted, possibly an entity. Though this client log doesn't seem to acknowledge the disconnect

Please provide debug.log from your client (more info on where it is and how to provide it in my signature below, just expand the spoiler)

Do you have any datapacks installed?

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

Do you get the same issue using Java 8 to run the server?

Can you provide the client log as well?

Please don't redistribute mods, if you need to distribute a modpack I recommend doing so with Curse

Remove OptiForge and Optifine. That's about all there is to it.

What do you have for antivirus?

Is this preventing you from playing or are you just concerned about it?

Show your Run.bat and server folder again

Are you renaming any files after reinstalling?

Skip

So at this point, go to %LocalAppData%\Temp and delete all folders starting with forge_installer, then repeat what I said last time.

There should be a debug.log... Did you only do what I said or did you do more than that?

The exact same log or does it say something else now?

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

Looks like something went wrong with the installation, delete the libraries folder and both jars and rerun the installer

Please provide debug.log from the server's logs folder (please use one of the paste sites listed in my signature below)

Does it say anything in the cmd window that pops up?