First, I have to correct myself, this is hardly a fix. Really, it's a work-around at best. So far, with some tests and net browsing, I've gathered a few interesting bits of info:
1) This bug hits several large mods, such as FOOK and Reborn.
2) It can sometimes be worked around by disabling the mod temporarily, although it didn't work for me.
3) I believe it's caused by a save-game corruption due to activating mods on an old game.
Now, you no doubt want to know what to do about it. The best solution is to start a new game with ALL the mods running that you want. Do NOT change any mods or you increase the risk of experiencing the issue. I did that and it seems to fix the problem (I cheated to level 20).
However, if you're really attached to your game, I will be uploading a mod patch called "No Level Up", which makes the max level 2, essentially preventing the level up process from happening and screwing up your game. You can then use cheats to upgrade your skills (if you wish). The cheat to use is "player.modav
I hope this clears up some confusion, sorry about this, but I'm certain it's not Reborn's fault, it's just shitty save game programming. Good luck,