HMPerson1

Forge 6.6.0.499 should fix it.

You have the Super Heros Mod installed. Remove it.

We have a subforum called Modder Support.

Copy ALL THE FILES into the jar.

Logs?

Rename the Forge zip to a jar.

That's the client log; we want the server log.

Bug in RedPower; not our problem.

Why did you need to downgrade to 1.0.0??

If you saved a world with certain mods installed, you MUST have at least those mods installed to play in that world.

It's a problem with one of your mods. Temporarily remove all of them add them back in one at a time to see which one causes the crash.

That's his problem, not ours.

What version are you running? Saying "latest" doesn't help when there are new versions every day.

Wrong forum.

It's an ID conflict between RedPower and Divine RPG. They're both occupying item slot 5007 (ID:5263), slot 5013 (ID:5272), slot 5015 (ID:5271), and slot 5021 (ID:5277). Look for those slots in your RP2 config, or turn on autoAssign.

What happens if you leave the IP field of server.prop empty?

mcp/conf/joined.srg

So...you've verified that this DOES NOT happen with a vanilla server and a vanilla client under the same circumstances?

Wrong section.

Make sure that your client configs match the server configs.

MultiMC is an open source Minecraft launcher. Look at how it launches Minecraft.

Update Super Mario mod.

MultiMC is an open source Minecraft launcher. Take a look at how it launches Minecraft.

Logs?

Check the firewall settings on both computers. Also, verify that this only happens with Forge installed and that it is not a vanilla issue.