I tried so many things but I can't seem the fix this.
I was playing the pixelmon modpack from curseforge, but i wanted to add my own mods that i have added to that modpack in my singleplayer game, so I swapped from that to Forge 1.12.2 (22.214.171.12460)
I added all the mods from my singleplayer folder (even nitpicked the specific versions so it was all IDENTICAL), and even though it works in singleplayer, it ALWAYS crashes before managing to start up in aternos.
I'm really not sure what to do now, the only thing I can think is that one (or more) of the mods are incompatible with servers as a whole??