Hi Ioannis,
Your question is similar to the one you posted here: openfast v2.3 - #5 by Jason.Jonkman. Again, it sounds like your model requires just about 2-GB of RAM and you are using the 32-bit version of OpenFAST. Reducing the memory requirements, e.g., by eliminating some analysis nodes, is likely the easiest solution if you can’t switch to the 64-bit version.
Best regards,