This error only occurs on server instances, I was never able to replicate it on a local singleplayer instance.

Forge version: 31.2.32
Mod List ripped straight from server logs:
forge-1.15.2-31.2.31-server.jar Minecraft {[email protected] DONE}
useful_backpacks-1.15.2-1.10.1.66.jar Useful Backpacks {[email protected] DONE}
repurposedstructures-1.15.2-1.7.2.jar Repurposed Structures {[email protected] DONE}
pandoras_creatures-1.15.2-1.0.0.jar Pandoras Creatures {[email protected] DONE}
Craft_and_Hunt_V1.1.0_MC_1.15.2.jar Craft and Hunt {[email protected] DONE}
mining-helmet-1.15.2-1.0.8.jar Mining Helmet {[email protected] DONE}
jei-1.15.2-6.0.2.12.jar Just Enough Items {[email protected] DONE}
Abnormals-Core-1.15.2-1.0.6.jar Abnormals Core {[email protected] DONE}
Buzzier-Bees-1.15.2-1.5.2.jar Buzzier Bees {[email protected] DONE}
HammerLib-1.15.2-15.2.3.jar HammerLib {[email protected] DONE}
Atmospheric-1.15.2-1.4.1.jar Atmospheric {[email protected] DONE}
Upgrade-Aquatic-1.15.2-1.7.1.jar Upgrade Aquatic {[email protected] DONE}
BetterCaves-1.15.2-1.0.jar YUNG's Better Caves {[email protected] DONE}
Waystones_1.15.2-6.0.2.jar Waystones {[email protected] DONE}
gravestone-1.17.5.jar Gravestone Mod {[email protected] DONE}
journeymap-1.15.2-5.7.0b3.jar Journeymap {[email protected] DONE}
autumnity-1.3.3-1.15.2.jar Autumnity {[email protected] DONE}
Placebo-1.15.2-3.1.0.jar Placebo {[email protected] DONE}
AutoRegLib-1.5-40.jar AutoRegLib {[email protected] DONE}
StorageDrawers-1.15.2-7.0.2.jar Storage Drawers {[email protected] DONE}
themidnight-0.5.9.jar The Midnight {[email protected] DONE}
betteranimalsplus-1.15.2-8.2.1.96.jar Better Animals Plus {[email protected] DONE}
WitherSkeletonTweaks-1.15.1-4.0.0.jar Wither Skeleton Tweaks {[email protected] DONE}
Reliquary-1.15.2-1.3.4.995.jar Reliquary {[email protected] DONE}
The-Endergetic-Expansion-1.15.2-v1.3.2.jar The Endergetic Expansion {[email protected] DONE}
snowundertrees-1.15.2-v1.1.1.jar Snow Under Trees {[email protected] DONE}
u_team_core-1.15.2-3.0.0.160.jar U Team Core {[email protected] DONE}
ProgressiveBosses-2.1.5-mc1.15.2.jar Progressive Bosses {[email protected] DONE}
AppleSkin-mc1.15.2-forge-1.0.14.jar AppleSkin {[email protected] DONE}
Swamp-Expansion-1.15.2-1.7.3.jar Swamp Expansion {swampexpansion@NONE DONE}
forge-1.15.2-31.2.31-universal.jar Forge {[email protected] DONE}
Quark-r2.1-245.jar Quark {[email protected] DONE}
Atum-1.15.2-2.1.3.jar Atum 2 {[email protected] DONE}
Charm-1.15.2-1.5.9.jar Charm {[email protected] DONE}
wendigoism-1.1.2.jar Wendigoism {[email protected] DONE}
dash-1.15.2-1.0.1.jar Dash {[email protected] DONE}
RottenCreatures-1.15.2-0.9.7.jar Rotten Creatures {[email protected] DONE}

When I replicated this error on my own server, just to see if this was an isolated instance, my servers console gave this error when I came close to the lighthouse structure:
[09:09:04] [Server-Worker-14/INFO] [STDOUT/]: [com.teammetallurgy.atum.world.gen.structure.lighthouse.LighthousePieces$LighthouseTemplate:spawnSunspeakers:79]: While [09:09:04] [Server-Worker-14/INFO] [STDOUT/]: [com.teammetallurgy.atum.world.gen.structure.lighthouse.LighthousePieces$LighthouseTemplate:spawnSunspeakers:97]: add sunspeaker [09:09:04] [Server-Worker-14/INFO] [STDOUT/]: [com.teammetallurgy.atum.world.gen.structure.lighthouse.LighthousePieces$LighthouseTemplate:spawnSunspeakers:79]: While [09:09:04] [Server-Worker-14/INFO] [STDOUT/]: [com.teammetallurgy.atum.world.gen.structure.lighthouse.LighthousePieces$LighthouseTemplate:spawnSunspeakers:97]: add sunspeaker [09:09:04] [Server-Worker-14/INFO] [STDOUT/]: [com.teammetallurgy.atum.world.gen.structure.lighthouse.LighthousePieces$LighthouseTemplate:spawnSunspeakers:79]: While [09:09:04] [Server-Worker-14/INFO] [STDOUT/]: [com.teammetallurgy.atum.world.gen.structure.lighthouse.LighthousePieces$LighthouseTemplate:spawnSunspeakers:97]: add sunspeaker [09:09:04] [Server thread/INFO] [minecraft/DedicatedServer]: [Sukiyu: Teleported Sukiyu to 192.5, 90.72269083441569, 176.5] [09:09:06] [Server thread/INFO] [minecraft/ServerPlayNetHandler]: Sukiyu lost connection: Disconnected [09:09:06] [Server thread/INFO] [minecraft/DedicatedServer]: Sukiyu left the game [09:09:09] [Server thread/INFO] [minecraft/ChunkManager]: ThreadedAnvilChunkStorage (atum): All chunks are saved

Just some debug code that I accidently left in, in the previous version - when fixing some Sunspeaker spawning issues. Doesn't mean anything is wrong :)

Whenever going near the lighthouse on the server, the server kicks you out with this error message: io.netty.channel.AbstractChannel$AnnotatedConnectException: Connection refused: no further information:
This only occurred after we went near the lighthouses.

Oh wait, I'm sorry! I grabbed the wrong error:
https://prnt.sc/tjdg7s

You can grab a build with the fix here: https://jenkins.girafi.dk/job/Team%20Metallurgy%20Mods/job/Atum%202/job/Atum%202%201.15.2%20Jenkins/
Will probably be a day or two, before it makes it into a release on CurseForge.