Bokob

I might be late to the party, but I might have found a fix to this bug. (and sorry for my bad english, not a native speaker) I had a couple players on my server that were having this issue. I tried a couple things without success. Since I was attempting to have to avoid completely resetting the server just for 2 people and I didn't want to remove the mods we were using. so I tried installing MC from scratch on my 2nd PC and oddly enough, I was having the same issue but my first PC was connecting fine. Copying .minecraft completely works but that's not a durable solution when I don't live in the same country of some of my member. I tried updating forge on both client and server side with no effects. Tried updating the mod, same thing. For some reason, I opened the config folder, which contained a lot of old mod and player info, so I deleted it server side. interestingly, now I could log on my 2nd PC but not of my first PC. This interesting reversal made me delete the config file on the client side (first PC) and voila! I troubleshooted all my users and they are all able to log in! TLDR, the server and the clients might need a config file cleanup when this happens. Stop the server/client, delete \.minecraft\config, launch server again. I hope that helps others!