Dear Joannes,
The FAST v8 S-Function for the MATLAB/Simulink interface is a discrete-time block, meaning that inputs go in at time step “n” and outputs come out at time step “n+1”. Within the block, the output calculation at n+1 involves an extrapolation of inputs (linear or quadratic), which is normally negligible when the inputs vary slowly in time, but is visible when the inputs vary a lot between time steps.
The error seems to imply that the model has going unstable, with vary large unrealistic displacement.
Best regards,