Dear Austin,
The failure of the BEM algorithm of AeroDyn v15.00 - v15.03 when Vx goes negative is a known issue. Working in collaboration with Envision, we have found the algorithmic problem causing the issue. While we have not yet publicly released the updated code, you can find a version of AeroDyn v15 with issue resolved here: github.com/NWTC/AeroDyn. Reverting to the older AeroDyn v14 will also work.
Bets regards,