We have investigated the second harmonic generation (SHG) signals of a guest/host system based on 4-(dimethylamino)-4’-nitrostilbene dissolved at 1 wt % in poly(isobutyl methacrylate) and poled in a sandwich configuration of ITO electrodes. By blocking the injection from the ITO surfaces with SiO coatings, a significant impact of charge carriers on the SHG decay pattern can be excluded. Using chloroform as a solvent, an additional contribution to the SHG decay kinetics is observed which is also active in the presence of the external electric poling field. Separating this effect from the pure orientational relaxation of the dopants leads to a highly improved reproducibility. Due to its weak temperature dependence, we tentatively ascribe this solvent-specific effect to the diffusion of dipolar impurities which may affect the local field factors and hyperpolarizability tensors. The resulting corrected SHG time constants related to the pure orientational relaxation still appear to be strongly decoupled from the alpha-process of the matrix as inferred from dielectric relaxation data of the bulk polymer. This basic discrepancy between SHG decay and alpha-process is found not to depend on film preparation, sample conductivity, sample geometry, and residual solvent content.