Spline IK and Auto-IK working together on the same bone chain could crash due to the somewhat hacky way that they were sharing the same list for the "iktree" forest. Resolved by doing what I should have done from the beginning, and made Spline-IK save its ik chains off into a separate list. While this fixes the crash, it might be worth investigating outright disabling this from working in this case, since it can be a bit confusing to have it appear to not work.