Dear David,
We are aware of a problem with the BEMT algorithm in AeroDyn v15.00 – v15.03 that manifests itself when Vx goes negative (common under operational conditions with low mean wind speed and high turbulence) – see the following forum post for a discussion on this and updated source code that addresses this issue: FAST v8 AeroDyn v15 convergence problems for low-speed, high-turbulence - #2 by Jason.Jonkman. In your case, however, I question whether your simulation is set-up correctly as the Vx reported in your error is well below zero and Vy is very large.
Best regards,