  Grain boundary self-diffusion of $^{57}$Co, $^{51}$Cr, $^{59}$Fe and
$^{54}$Mn in a coarse-grained, single-phase fcc CoCrFeMnNi high entropy alloy
is measured in a wide temperature range of 643 to 1273~K in both C- and B-type
kinetic regimes after Harrison's classification. The results suggest that the
product of the pertinent segregation factors, $s$, and the grain boundary
width, $\delta$, is about 0.5~nm for all elements at temperatures $T>800$~K.
Whereas one short-circuit contribution is observed at higher temperatures above
800~K, the penetration profiles in the C-type kinetic regime (643 -- 703~K)
reveal two distinct contributions that hint towards a phase decomposition at a
fraction of high-angle grain boundaries at these temperatures. A correlative
microscopy combining transmission Kikuchi diffraction and atom probe tomography
manifests formation of neighbouring Ni-Mn-rich and Cr-rich precipitates at a
segment of high angle grain boundaries. Transmission electron microscopy
revealed an increased dislocation density in the vicinity of such interfaces
which is suggested to be a reason of the enhanced diffusion rates at low
temperatures for such short circuits.
