Free Size Optimization with interface laminates

Hello everyone,

I am currently working on an free size optimization of a wing consisting of a laminated top and bottom half.

Therefore i am pursuing a ply-based approach. When adding an interface laminate to my design variable the optimization terminates with the following error:

Is there any known solution for this issue? If not, what is the explanation behind it?

I have added  my provisional model, if someone wants to take a look at it. 

It contains some dummy material properties/load cases yet, but if some other major modeling mistake stand out to you, i would be glad if you share them with me.

Thank you in advice for your help!

Best regards,

Lukas

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