I have nooo idea whats wrong
Neither have we. You have not provided even a shred of useful info, beginning with which version of AE, system info, kind of project and so on.
Im running Adobe After effects CC 2015.3, I have an Intel core i5-4690K, 16GB of DDR3 1866 memory, and a GTX 980, my hardware is more than enough to run after effects, The project is a Video intro that kind of "broke" somehow, Iv'e tried methods on youtube to remove the X and none of them work, they just turn the X black.
Please provide a screenshot of this mysterious X. show the whole interface so we may take a gander
The X is probably from an unlicensed plugin. You likely have Particular, Shine, or some other Trapcode plugin in the scene. They allow you to use the plugin for demo purposes, but you must purchase the plugin and license it to remove the x.