Dear @Bart.Klootwijk,
It sounds like your model is numerically unstable such that reducing the OpenFAST time step (DT) should solve the problem, considering that you’ve already set appropriate values for NumCrctn, DT_UJac, and UJacSclFact. Can you summarize your solver settings?
Regarding (2), I’m not aware of any discrepancies between the WAMIT and HydroDyn inputs, but I was not the one to make this model either. You can find the WAMIT input files for the OC4-DeepCwind semisubmerisble in my post dated May 15, 2023, in the following forum topic: OC4-DeepCwind semisubmersible - WAMIT Files.
Best regards,