Used Advanced Enchanting Table. Brought down to level 16. Found I couldn't gain any exp whatsoever even though I was picking up a lot of it. Killing mobs, mining, cooking, nothing. No levels, the exp bar doesn't even fill up. Even giving myself exp bottles does nothing.

I am running Enchanting Plus 4.1.0.115 alongside EnderIO 3.1.156

Killing myself fixed it. Still not sure what's wrong.

It seems to be caused by repairing items, once your XP Bar reaches nothing, instead of bringing you down a level, the XP Bar's NBT value ("XpP" - inside your player .dat file) goes into the negatives, producing this weird glitch of not being able to get experience.
(I tested this multiple times with and without other mods installed)
Serious bug IMO, +1 please fix.

I did a little digging and found that the mod is using player.addExperience(-cost) to do the experience adjusting, but according to this forge thread, they say it might not be the right thing to do.
Considering I'm a mere java fetus and have no idea how much of it works I might be wayyy off, and I don't want to belittle the dev's knowledge, but if I don't try I don't learn :3

Edit: Check out this • http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/modification-development/2503178-remove-xp-from-player-forge?comment=4

This issue has been fixed in the 1.12.2 version of the mod.