Dear Charlie,
The AeroDyn convergence issues that were common in AeroDyn v15.03 of FAST v8.16 have been resolved in OpenFAST v1.0.0 (or newer), so, you could always upgrade to see if that solves your problem. However, I recall that the convergence issues happened most frequently at low wind speeds with high turbulence, which is not where you are seeing issues.
Otherwise, I see large deflection warnings in your simulation, which I would guess in your case are caused by the use of improper initial conditions. As discussed several times on this forum, it is important to set at a minimum the initial conditions of rotor speed and blade-pitch angle appropriate to each mean wind speed. It doesn’t sound like you’ve calculated/applied these for the WindPACT 1.5-MW turbine (and I don’t have them myself), but the process for calculating them has been discussed on our forum…e.g. here: WP 1,5 MW turbine - Pitch Angle - #2 by Jason.Jonkman.
Best regards,