Fix "Attempted to load class for invalid dist DEDICATED_SERVER"

[main/ERROR] [net.minecraftforge.fml.loading.RuntimeDistCleaner/DISTXFORM]: Attempted to load class net/minecraft/client/gui/screens/Screen for invalid dist DEDICATED_SERVER

NeoForge writes the same line with net.neoforged.fml.common.asm.RuntimeDistCleaner at the start. DEDICATED_SERVER is the loader's name for a server without a screen, and invalid dist means the class that was asked for belongs to the other side: the game on a player's computer.

1. On the Files tab, open the newest file in the crash-reports folder, or scroll up in the Console from the error.
2. Look for Mod file: followed by a path in /mods. That is the jar to remove.
3. No path? When the class is in a mod's own package, such as com/author/somemod/..., the package names the mod.
4. Still unsure? Move half of the mods out of mods and start the server. If it starts, the client mod is in the half you moved; if it crashes again, it is in the half that stayed. Keep halving that half.

Halving a modpack's mods can give missing dependency errors along the way. They only mean a library went out without the mod that needs it, or the other way round. Put it back and carry on; missing mod dependencies explains those lines.

Once you know the jar, remove it with Filter installed mods on the Mods tab, or delete it from mods on the Files tab, and start the server again. A second client mod in the same folder shows up as the same error with another class; repeat until the server starts.

These kinds of mods are made for the game, not for the server. Players can use them on any server, yours included, without the server having them.

• Shader and rendering mods, such as OptiFine, Oculus, Iris, Embeddium and Sodium
• Minimaps and world maps, unless the mod page says it has a server part
• Zoom, freecam, HUD, menu and key binding mods
• Sound, animation and particle mods that only change what one player hears or sees

Mods that add blocks, items, mobs, dimensions or recipes are the opposite: they must be on the server and on every player. When you are not sure, the mod's page on Modrinth says whether it is needed on the client, the server or both. The catalog on the Mods tab only lists mods that run on a server.

A modpack is made for players, so its folder on your computer is full of client mods. Copy that folder to a server and this crash follows. Install the pack on the Mods tab instead: the panel sets it up from the pack's server files, without the mods that are only meant for players. Install a modpack shows the steps.

Fabric leaves out mods that declare themselves client only, so those never cause trouble there. A client class that still gets loaded stops the server with Cannot load class ... in environment type SERVER. The fix is the same: find the mod and take it off the server.