Please check the material fitting to make sure its wavelength range is fixed as expected.
Also please check two wavelengths (two end-values) and make sure the mesh size is the same.
Then most likely due to
1: PML too thin. Please use more number of layers, say 64; you may also use "steep angle PML". Make sure the PML has about half wavelength thick, and at least half wavelength away from the structure. You can do 2D simulation.
2: significnatly increase simulation time and reduce autoshutoff min.
3: make sure the silver is well resolved by mesh. You can use override mesh for the silver layer with mesh size 5nm.