DaemonUmbra

Is this on your own computer or are you using a hosting company? Are both the minecraft and forge jar files in the same folder?

You are not modifying your minecraft server jar in any way, correct?

Just looked a few posts back, try putting the server in a place on your computer that doesn't have accent marks in the path. Maybe rename "Área de Trabalho" to "Area de Trabalho" Nevermind, I assumed this was an issue when I had forgotten one other thing in my test.

Hold on a sec, just to verify are we running a server or a client? OCT's instructions are for a client, not a server.

You're not renaming the minecraft server jar are you?

Well you're not far from it, see the "files" button at the top of the forums? That's it.

Just to make sure, where are you getting your forge?

Remove Optifine and post the updated log I just noticed the same as OP below...

Have you tried re-running the installer? Delete the minecraft_server.1.12.2.jar and try that

How did you set up the server?

You should, yes

What it looks like is Adventure Bags is broken, trying to access client-only code on the server side.

Please run it from the command line and show what it spits out.

I can tell you that the FMLHandshakeMessage class the error originates from has not been changed at all between those two versions (according to a git diff check between the two commits)

Thaumcraft's particle engine is broken at the moment it seems: https://github.com/Azanor/thaumcraft-beta/issues/858

The posted log has nothing to do with a magical forest and involves mods that I do not see in your screenshot, please verify the posted log is the correct one and if not, please post the correct one in a new reply.

Lex knows infinitely more than me in this matter and as far as I can tell with my novice experience the handshake is where this issue begins. I'm sorry but it appears your "crucial" mod is the source of your problems.

If you did remove your coremods you posted the wrong log

To my knowledge purchasers of Minecraft: Bedrock Edition are not granted copies of Java Edition, however purchasers of Java Edition do get a code redeemable on the Microsoft Store for Bedrock Edition, so I'm sorry to say that you will have to pay for Java Edition separately, and you would have gotten both if matters were the other way around.

Thaumcraft is broken

Don't unzip mods, even forge should stay as a jar

Pixelmon are saying that because "it worked before", they need to update their mod with a more recent MDK.

@MDW01 Didn't we see this same error in another thread?

I found the issue and we should both be embarrassed...you for the mistake and me for the amount of time it took for me to find it, I have submitted a Pull Request to your first repo with the fix and formatted the repo in the way that is intended from the MDK package. The issue: Both of your proxies were the "common proxy", speaking of which, from the common issues thread:

Your repo doesn't have your whole workspace in it, this can sometimes get in the way of reproducing errors, I would recommend using a git client to upload every file that it will let you (one of the files provided by Forge's MDK is a .gitignore file, this will automatically determine which files do not need to be uploaded, you will not need to do this manually).