Upon logging into a server, the following issue occurs and the user is kicked out.
https://pastebin.com/Yz1eRnGK

Forge 2796
In Game Info 2.8.2.94-universal
In Game Info - Serene Seasons 1.0.0.1
Using Optifine

I am having the same issue, sometimes players get kicked other times it take a few. Exact same console log information.

Relevant information:

Server Details:
OS: Linux Ubuntu
Java: OpenJDK 1.8.0_222
Allocated ram amount: 10 Gigs
Forge Version: 2838
Modpack in use: Rougelike Adventures and Dungeons
Link to Modpack: https://www.curseforge.com/minecraft/modpacks/roguelike-adventures-and-dungeons
Copy of latest.log file:
latest.log

I witnessed it happen when my wife logged in. Her system is as follows:

Client details:
OS: Windows 10
Java: Java 1.8.0 Update 51 (this is what ships with Twitch by default)
Allocated Ram: 6144MB
Forge Version: 2838
Modpack in use: Rougelike Adventures and Dungeons
Link to Modpack: https://www.curseforge.com/minecraft/modpacks/roguelike-adventures-and-dungeons
Optifine in use: No

If further information is required please let me know and I will do my best to get it for you.