Tyrion S.

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Hi, I stumbled across this issue. I was able to replicate it with Forge 1.20.2, 1.20.1, 1.20, 1.19.4, didn't test further back. When trying to connect to a server, doesn't seem to matter if it's with BungeeCord, Velocity, just Paper or a plain vanilla server, it cannot auth with Mojang if this setting is enabled. I am not using a proxy or VPN and I wasn't able to replicate this with the client being Vanilla/OptiFine/Fabric/Feather/LabyMod. Vanilla server says "tried to join with an invalid session" and "Failed to verify username", Velocity and BungeeCord both give issues related to not being able to authenticate the session/account. No errors in the client- or server log. Only potentially helpful thing I can add is that, with a fresh MC install or MultiMC instance PLUS forge install, it works the first time the game is started and only then. This is on Windows 10 with Java 17 and 19, the standard launcher and Prism, a MultiMC fork, no Forge mods in use

double check you don't have prevent-proxy-connections set to true in server.properties. I have no idea WHY this is a problem with forge, but I can 100% replicate it with different servers, forge installs and computers.