All parties must install a fresh and completely new version of forge AND the mod(s) that they have installed. I noticed that my friend's launcher said Forge 28.0.75 (I think) and mine said Forge 28.0.5. I installed the newest version on both computers, relaunched, and it worked! Hope this helps, it did for me!
v_veX101
I found the solution!
All parties must install a fresh and completely new version of forge AND the mod(s) that they have installed. I noticed that my friend's launcher said Forge 28.0.75 (I think) and mine said Forge 28.0.5. I installed the newest version on both computers, relaunched, and it worked! Hope this helps, it did for me!