One player cannot connect to Fabric backend servers despite using the exact same MC instance as everyone who can

Expected Behavior

All players can connect

Actual Behavior

One player cannot connect - he gets stuck on loading world then gets the error message 'Unable to connect you to Lobby. Please try again later.' This still happens even if I change Velocity to make the default server the 2nd backend server ('Creative', using the same modpack). The player is using the exact same MC instance that everyone who CAN connect is using i.e. I exported my (working) instance as a zip, sent it to him, he imported it and ran it and it fails in the same way.

I've attached the relevant snippets from the Velocity and backend server consoles, and the client's logs.

backend Lobby console.txt

Velocity console.txt

client log (Minecraft).txt

client log (Prism launcher).txt

Steps to Reproduce

1. Install my MC instance
2. Try and join my server
3. Be this specific player

Plugin List

Velocity: velocity, luckperms, vpacketevents, signedvelocity, carbonchat, proxy-command, rewhitelist, voicechat

Fabric 1.21.1 backend:
accessible-step-fabric-mc1.21.1-2.1.0+1.21.1.jar
additionallanterns-1.1.2-fabric-mc1.21(1).jar
amendments-1.21-1.2.24-fabric.jar
architectury-13.0.8-fabric.jar
athena-fabric-1.21-4.0.2.jar
bettercommandblockui-0.5.2a-1.21.jar
BuildingWands-fabric-MC1.21-2.9.jar
Camerapture-1.10.6+mc1.21.1-fabric.jar
carbonchat-fabric-3.0.0-beta.32.jar
chipped-fabric-1.21.1-4.0.2.jar
cloth-config-15.0.140-fabric.jar
collective-1.21.1-8.3.jar
connectedglass-1.1.14-fabric-mc1.21.jar
coords_copy-1.0-1.21.1.jar
crossstitch-0.1.6.jar
DiagonalWindows-v21.1.1-1.21.1-Fabric.jar
doubledoors-1.21.1-7.1.jar
doubleslabs-fabric-1.21.1-0.3.0.jar
DramaticDoors-NeoQuiFab-1.21.1-3.3.2.jar
elevatorid-fabric-1.21-1.11.1.jar
emi-1.1.22+1.21.1+fabric.jar
emotecraft-for-MC1.21.1-2.4.10-fabric.jar
Essential_1-3-9_fabric_1-21-1.jar.disabled
fabric-api-0.116.5+1.21.1.jar
fabric-language-kotlin-1.13.4+kotlin.2.2.0.jar
FabricProxy-Lite-2.9.0.jar
FastItemFrames-v21.1.4-1.21.1-Fabric.jar
FlowerPatch-fabric-1.21.1-7.1.0.jar
ForgeConfigAPIPort-v21.1.4-1.21.1-Fabric(1).jar
fusion-1.2.10-fabric-mc1.21.jar
halfdoors-1.21.1-1.0-SNAPSHOT.jar
instantgroup-fabric-1.21.1-1.7.0.jar
iris-fabric-1.8.8+mc1.21.1.jar
jei-1.21.1-fabric-19.22.1.316.jar
LuckPerms-Fabric-5.4.140.jar
malilib-fabric-1.21-0.21.9.jar
mcw-bridges-3.1.0-mc1.21.1fabric.jar
mcw-doors-1.1.2-mc1.21.1fabric.jar
mcw-fences-1.2.0-1.21.1fabric.jar
mcw-furniture-3.3.0-mc1.21.1fabric.jar
mcw-lights-1.1.2-mc1.21.1fabric.jar
mcw-paths-1.1.0fabric-mc1.21.1.jar
mcw-roofs-2.3.2-mc1.21.1fabric.jar
mcw-stairs-1.0.1-1.21.1fabric.jar
mcw-trapdoors-1.1.4-mc1.21.1fabric.jar
mcw-windows-2.4.0-1.21.1fabric.jar
Measurements-fabric-1.21.1-3.0.1.jar
modern-lights-2.5.0+[1.21-1.21.1].jar
modmenu-11.0.3.jar
moonlight-1.21-2.20.7-fabric.jar
noclip-1.3.2+1.21-fabric.jar
player-animation-lib-fabric-2.0.1+1.21.1.jar
proxy-command-fabric-0.jar
PuzzlesLib-v21.1.38-1.21.1-Fabric.jar
rechiseled-1.1.6b-fabric-mc1.21.jar
resourcefullib-fabric-1.21-3.0.12.jar
Searchables-fabric-1.21.1-1.0.2.jar
SignedVelocity-Fabric-1.3.0.jar
sit-1.21-28.jar
sodium-fabric-0.6.13+mc1.21.1.jar
sodiumextras-fabric-1.0.8-1.21.1.jar
supermartijn642configlib-1.1.8-fabric-mc1.21.jar
supermartijn642corelib-1.1.18a-fabric-mc1.21.jar
supplementaries-1.21-3.4.4-fabric.jar
trowel-1.4.0.jar
tweakeroo-fabric-1.21-0.21.59.jar
voicechat-broadcast-fabric-1.1.0-1.21.1.jar
voicechat-fabric-1.21.1-2.5.36.jar
worldedit-mod-7.3.8.jar
xercapaint-1.21.1-1.0.1.jar

Velocity Version

Velocity 3.4.0-SNAPSHOT (git-49e2988e-b525)

Additional Information

No response

Start by comparing the attached Velocity console, backend Lobby console, client Minecraft log, and client Prism launcher log around the failed connection. Trace the connection from the client through Velocity to the Fabric backend and compare it with a successful player; done means isolating a reproducible cause for this player-specific failure and documenting the verified resolution.