Dear Francisco,
I know this is an old thread, but the source of this problem has now been found and fixed in OpenFAST. The problem was related to the storing of the time variable in MoorDyn in single precision, instead of double. This problem has now been fixed in a recent pull request OpenFAST: github.com/OpenFAST/openfast/pull/486. This problem was also recently discussed on our forum–see: Question #1 for turbulence fields (points outside of field).
Best regards,