Server Problem

I recently created a modded server to play with some of my friends. I turned on Cracked so my friends would be able to join as well as myself since we all play with cracked launchers, I was able to join my server but my friend was not, despite having the exact same modpack as myself he kept being the "Internal server error" message whenever he attempted to join my server and I couldn't find any thread on this. I apologize if this has been answered already but i'd still request some help.