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When M'khiin joined my party, she was unable to level up, as the next level up was set for -1 XP. The menu said she was ready to level up, but the level up button was grayed-out.
I already solved it; I'm just posting here to help anyone who has the same problem. The cause is the XPLEVEL.2da file, which currently is missing a line for Shamans. Unzip the attached .zip file into your override and M'khiin will be able to level up normally.