Dear Daniel,
Setting SkewMod to 1 should not cause any problems.
While you may have disabled all structural DOFs in ElastoDyn, you said that you started with Test26, which uses BeamDyn to model the blade structural dynamics. So, my guess is that you still have an aero-elastic solution. Regardless, recompiling FAST using the source code referred to in the forum topic I linked above (FAST v8 AeroDyn v15 convergence problems for low-speed, high-turbulence - #2 by Jason.Jonkman) should solve the problem. Alternatively, if you are interested in pure aerodynamic response, without the influence of structural DOFs, I suggest that you use standalone AeroDyn v15.
Best regards,