The spin Hall effect in a heavy metal thin film allows to probe the magnetic properties of an adjacent magnetic insulator via magnetotransport measurements. Here, we investigate the magnetoresistive response of yttrium iron garnet/platinum heterostructures from room temperature to beyond the Curie temperature TC≈560K of the ferrimagnetic insulator. We find that the amplitude of the (local) spin Hall magnetoresistance decreases monotonically from 300K towards TC, mimicking the evolution of the saturation magnetization of yttrium iron garnet. Interestingly, the spin Hall magnetoresistance vanishes around 500K, well below TC, which we attribute to the formation of a parasitic interface layer by interdiffusion. We confirm the presence of such an interface region with Fe and Pt intermixing by transmission electron microscopy and spatially resolved energy dispersive x-ray analysis. Around room temperature the nonlocal magnon-mediated magnetoresistance exhibits a power law scaling Tα with α∼3/2, as already reported. The exponent decreases gradually to α∼1/2 at around 420K, before the nonlocal magnetoresistance vanishes rapidly at a similar temperature as the spin Hall magnetoresistance. We attribute the reduced α at high temperatures to the increasing thermal magnon population which leads to enhanced scattering of the nonequilibrium magnon population and a reduced magnon diffusion length. Finally, we find a magnetic field independent offset voltage in the nonlocal signal for T>470K, which we associate with electronic leakage currents through the normally insulating yttrium iron garnet film. Indeed, this nonlocal offset voltage is thermally activated with an energy close to the band gap.