Dear Daniel,
If you disable ServoDyn, you should disable both the generator degree of freedom (DOF) and the nacelle-yaw DOF. You’ve disabled the generator DOF, but didn’t mention the yaw. My guess is you are modeling a free-yaw machine, which may be the source of the problem (i.e. nacelle yaw may be driving an instability).
Regardless, the error regarding “no valid value of phi” has recently been resolved through an algorithmic change to the BEM algorithm of AeroDyn v15, as reported in the following forum topic: FAST v8 AeroDyn v15 convergence problems for low-speed, high-turbulence - #2 by Jason.Jonkman.
Best regards,