So I've made an inserter but it acts weird, it adds 1 new stack and adds 1 item to an already existing one, the code responsible for this:

	public void addItemToChest(BlockPos pos, World worldIn, Entity entityIn) {
	if (!worldIn.isRemote) {
		IBlockState state = worldIn.getBlockState(pos);
		Block block = state.getBlock();
		EntityItem entityItem = (EntityItem) entityIn;
		if(block.equals(Blocks.CHEST)) {
			BlockChest chest = (BlockChest) block;
    		ILockableContainer container = chest.getContainer(worldIn, pos, false);
			for (int i = 0;i < container.getSizeInventory();i++) {
				if (container.getStackInSlot(i) != null) {
					if (container.getStackInSlot(i).getItem().equals(entityItem.getEntityItem().getItem())) {
        				if (container.getStackInSlot(i).stackSize < 64) {
        					container.getStackInSlot(i).stackSize++;
        					entityItem.setDead();
        				}
        			}
    			} else {
    				container.setInventorySlotContents(i, entityItem.getEntityItem());
    				entityItem.setDead();
    				break;
    			}
			}
		}
	}
}

So when I drop 1 item on top of the inserter it adds it to all the stacks of that item and makes a new stack of the item

it worked before so...

You're a freaking idiot. Seriously.

 

if (container.getStackInSlot(i).stackSize < 64) {
container.getStackInSlot(i).stackSize++;
entityItem.setDead();
}

 

Seriously.  What the hell.  All of your problems are here.

 

And why are you not using Capabilities for item storage?

I don't know how to use them and never used them

So learn. They aren't that hard.

https://mcforge.readthedocs.io/en/latest/datastorage/capabilities/

Oh wait, would it work like this:

public void addItemToChest(BlockPos pos, World worldIn, Entity entityIn) {
	if (!worldIn.isRemote) {
		IBlockState state = worldIn.getBlockState(pos);
		Block block = state.getBlock();
		EntityItem entityItem = (EntityItem) entityIn;
		ItemStack stack = entityItem.getEntityItem();
		Item item = stack.getItem();
		if(block.equals(Blocks.CHEST)) {
			BlockChest chest = (BlockChest) block;
    		TileEntityChest tile = (TileEntityChest) worldIn.getTileEntity(pos);
    		IItemHandler handler = tile.getCapability(CapabilityItemHandler.ITEM_HANDLER_CAPABILITY, (EnumFacing) state.getValue(FACING));
			for (int i = 0;i < handler.getSlots();i++) {
				if (handler.getStackInSlot(i) != null) {
					if (handler.getStackInSlot(i).getItem().equals(item)) {
        				if (handler.getStackInSlot(i).stackSize < stack.getMaxStackSize()) {
        					handler.getStackInSlot(i).stackSize++;
        					entityItem.setDead();
        				}
        			}
    			} else {
    				handler.insertItem(i, stack, false);
    				entityItem.setDead();
    				break;
    			}
			}
		}
	}
}

Run it and see.

 

No.

 

Because you didn't fix the issue I point out in the code I actually copied.  You changed a line I didn't copy.

 

Can you tell me what exactly was the issue

Look at the chunk of code and manually step through it. Use the debugger if you have to.

 

The error it obvious.

I still don't get it...

OOOHHHH I need to break