Dear Rene,
As first reported here: FAST v8 AeroDyn v15 convergence problems for low-speed, high-turbulence - #2 by Jason.Jonkman, working in collaboration with Envision Energy, we have found the algorithmic problem causing the convergence issues in AeroDyn v15.00 - v15.03 when Vx goes negative. While we have not yet publicly released an update to FAST with this version of AeroDyn, you can find AeroDyn v15.04 with this issue resolved here: nwtc.nrel.gov/AeroDyn.
Bets regards,