Dear Jannik,
I only looked briefly at your input files and output results, but I believe the problem is your time step (DT) is too large for the system frequencies you are resolving. Rules of thumb for selecting the aerodynamic and structural time steps are summarized in the following forum topic: FAST Integrator, step size & precision - #2 by Marshall.Buhl. I haven’t tried to calculate your required structural time step, but for a rotor spinning at 500 rpm (8.33333 Hz), to get 200 azimuth steps per revolution would require a time step of:
DT = 1/(200*8.33333) = 0.0006 s.
Best regards,