  We consider an open quantum system in contact with fermionic metallic
reservoirs in a nonequilibrium setup. For the case of spin, orbital or
potential fluctuations, we present a systematic formulation of real-time
renormalization group at finite temperature, where the complex Fourier variable
of an effective Liouvillian is used as flow parameter. We derive a universal
set of differential equations free of divergencies written as a systematic
power series in terms of the frequency-independent two-point vertex only, and
solve it in different truncation orders by using a universal set of boundary
conditions. We apply the formalism to the description of the weak to strong
coupling crossover of the isotropic spin-1/2 nonequilibrium Kondo model at zero
magnetic field. From the temperature and voltage dependence of the conductance
in different energy regimes we determine various characteristic low-energy
scales and compare their universal ratio to known results. For a fixed finite
bias voltage larger than the Kondo temperature, we find that the
temperature-dependence of the differential conductance exhibits non-monotonic
behavior in the form of a peak structure. We show that the peak position and
peak width scale linearly with the applied voltage over many orders of
magnitude in units of the Kondo temperature. Finally, we compare our
calculations with recent experiments.
