In some case, some Mod Gui cannot be opened in a multiplayer session. It does not crash the game, but something like the following is printed on the client's log:
Trying to open invalid screen with name: Potion Bag. Closer examine shows that the msg is printed by net.minecraft.client.gui.screen.Screens.open(@Nullable ContainerType<T> type, MinecraftClient client, int id, Text name). Its value comes from OpenContainerS2CPacket:

   public ContainerType<?> getContainerType() {
      return (ContainerType)Registry.CONTAINER.get(this.containerId);
   }

This mod (https://github.com/zabi94/ExtraAlchemy) uses the ScreenHandlerRegistry in a very standard way. The ScreenHandlerType variable POTION_BAG is a static variable and statically initialized. The problem will happen if the POTION_BAG is not accessed during the mod initialization. By adding zabi.minecraft.extraalchemy.screen.ModScreenHandlerTypes.POTION_BAG.toString(); to the end of the mod initializer, the problem is solved.

I would suggest that the ScreenHandlerType variables should be initialized in the mod initializer, it should not be statically initialized.

zabi94/ExtraAlchemy@5e4ce61

zabi94/ExtraAlchemy#87

Extra Alchemy dev here. It's not an api problem, statically initializing things and assigning the value through a registration isn't exactly what I'd define standard.
It's fixed already in the mod by using a method called during mod init to register and assign the values.

This issue can be closed.

Ideally you'd called some empty no-op method to load the class instance to register the screen handler types.

I do this in a single init method I call in my mod which then loads everything from all registries: https://github.com/i509VCB/BulkyShulkies/blob/1.16.x/src/main/java/me/i509/fabric/bulkyshulkies/registry/ShulkerRegistries.java

Will close this since it's a static init issue on the side of user error