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This approach screams "static initializers" and static initializers are bad. Just instantinate your blocks/items/whatever directly in the registry event. Not in a static initializer, not in FML events. You are also doing extra unnecessary work, think about it. Thirst you instantinate your stuff somewhere, then you add them to the set in the constructor, then you loop through the set in the event. That's at the very least 3 lines of code per block. Now think of the recommended approach. You instantinate your stuff directly in the registry event as an argument passed to register. That's one line of code per thing. With the recommended approach you are doing only 33% of the work. Also CommonProxy makes no sense. Proxies are for separating sided only code. Common code goes whereever but your proxy.

I was mistaken at first. Server clearly loads all the mods as you can see here: [21:26:40] [Server thread/INFO] [FML]: Attempting connection with missing mods [minecraft, mcp, FML, forge, aether_legacy, biomesoplenty, lucky, orbis_api] at CLIENT [21:26:40] [Server thread/INFO] [FML]: Attempting connection with missing mods [minecraft, mcp, FML, forge, aether_legacy, biomesoplenty, lucky, orbis_api] at SERVER

I believe you need to run the forge-1.12.2-(numbers)universal.jar

We can't help you without your code. Although from the error trace I suspect that you haven't registered your IGuiHandler. Also any particular reason you are using 1.10.2? You should be using 1.12.2.

also works with assets\minecraft\textures\gui\mojang.png (but not in all forge versions)