so I changed from calling directly
Minecraft.getMinecraft().effectRenderer.addEffect(new EntitySweepAttack2FX(Minecraft.getMinecraft().getTextureManager(), player.worldObj, player.posX + dd0, player.posY + (double)player.height * 0.5D, player.posZ + dd1, 0, 0, 0));
, to making a method that simply calls this line but the difference is that I added the @SideOnly(Side.CLIENT) annotation, and the error is gone so I guess that's good. Didn't do anything in the proxy though