The nonlinear AC magnetic response of superconductors in mixed (vortex) state is not completely understood. The simple extension of standard flux-creep models at short-time scales is unable to explain several peculiar effects, such as the presence of unexpectedly high vortex activation energies in the AC magnetic signal close to the DC irreversibility line. By investigating the AC magnetic response of superconducting YBCO films with a well characterized pinning structure and YBCO/PrBCO superlattices, as model samples, we show that close to the DC irreversibility line this corresponds to a non-diffusive, thermally activated vortex hopping over a length which overcomes the mean distance between the pinning centres, and the large vortex activation energy is the result of a pinning-enhanced viscosity of the vortex system. This suggests a new approach for the existence of a pinned vortex liquid in superconductors.