Nevermind. Found the problem. I did not removed all the mods cleanly. There is one mod that slip my eye causing the mismatching of mods on server side and client side. I can get in just fine now. You can close this thread now. 
Posts by MatthewAlden
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I have a problem. I played on the server with a mods that adds items but don't add any blocks onto the world. But, i'm having performance issue so i decided to remove the mods. Now, the server starts just fine but refuse to let me in. The console said mismatch mods but i had removed the mods off the server and my mods folder. The worst thing is the world backup decided to backup on top of my already working pre-modded world. Now, i can't get in my own server and i had read a couple forum post but removing config file on both server and client doesn't bear fruit. I have seen around about a way to clear out leftovers from mods but can't figure a way to do that on aternos.