Through a combination of neutron scattering, dielectric spectroscopy, and rheological measurements we study the impact of purely intramolecular cross-linking on a melt fully made of polymeric single-chain nanoparticles (SCNPs)-a novel class of ultrasoft nano-objects. While the alpha-relaxation is unaffected with respect to the reference melt of linear chains, the emerging polymer/colloid duality of SCNPs leads to the almost complete smearing out of the rubbery plateau. This is the opposite effect to the creation of a permanent 3D network by intermolecular bonds. In addition, neutron scattering shows that a new relaxation mechanism slower than the alpha-relaxation appears at intermediate length scales. These are beyond the interchain distance but yet far from the hydrodynamic regime. This new slow relaxation-also detected by dielectric spectroscopy-contributes to the hierarchy of processes needed for the full relaxation of the SCNP melt and is tentatively related to the heterogeneities provoked by the internal multiloop topology of the SCNPs and the segregation of their internal domains.