@Dmytro Fedin,
Please configure "RCWA solver" with 4 threads if you do not have Ansys HPC licenses on your server.
>>> Resource configuration elements and controls – Ansys Optics 
If you check run with design environment (default) in the resources configuration, set Threads in the Design environment tab to 4. 
Otherwise, if you have Ansys HPC licenses, set the threads according to the number of HPC license available on the server.
>>> Lumerical solve, accelerator and Ansys HPC license consumption – Ansys Optics
>>> Ansys HPC Licensing Calculator
If you are an Ansys Channel Partner, kindly register for support with Ansys Optics/Lumerical for access to Commercial Technical Support (email/phone). >>> How to register for support with Ansys Lumerical – Ansys Optics.
Checkout the Ansys Optics/Lumerical – FAQs for answers to common and known issues. Thank you!
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