I haven't seen this be reported anywhere at all, but whenever I click the codex, it turns black and says "there is nothing more to be learned about this biome" without adding anything to the biome it was meant to be. It looses it's biome too. Right clicking cave tablets also turns them black. To my understanding this is caused by invalid NBT data. I hadn't had this issue the day before, and added a couple of mods in the meantime (being a portable spawner mod, some enchant mods and a horse genetics mod).

I found its an error with a mod called "mo' enchantments".