*** ERROR # 735 *** Negative modal damping obtained from TABDMP1

Hello Luca, 

This is probably related to the extrapolated values found in the frequency range not defined by you directly in TABDAMP1. For example, your damping curve goes only up to 200Hz and your modal space up to 600Hz and when OptiStruct extrapolates the damping curve to 600Hz, it finds negative values. Try to add one extra line in the TABDMP1 curve covering higher frequencies and setting a positive damping value and the model should work fine. 

Thanks,
Vinicius

Thank you so much 

Could it be related to the amount of eigenvalue i've asked him to compute in relation to the frequency band my tab cover?

Yes, try to extend your tab up to the highest frequency in your eigenvalues extraction.