Dear Yaakov, thank for your answer.
I already checked the 'angle comand' and the 'vect orth' comands by considering different values of the rotation angle. Both comands work if a linear elastic analysis is performed. However I read in the user-manual that 'if the axes differ from the material coordinate axes a VECTor ORTHotropic record should be specified' for orthotropic plasticity. Furthermore, according your suggest, I tried to replace the 'vect orth' comand with the 'angle' one (attached file), but I had the same problem when the Hill plasticity model is activated.
Best,
Cri