  We theoretically study the propagation of an interacting Bose-Einstein
condensate in a two-dimensional disorder potential, following the principle of
an atom laser. The constructive interference between time-reversed scattering
paths gives rise to coherent backscattering, which may be observed under the
form of a sharp cone in the disorder-averaged angular backscattered current. As
is found by the numerical integration of the Gross-Pitaevskii equation, this
coherent backscattering cone is inversed when a non-vanishing interaction
strength is present, indicating a crossover from constructive to destructive
interferences. Numerical simulations based on the Truncated Wigner method allow
one to go beyond the mean-field approach and show that dephasing renders this
signature of antilocalisation hidden behind a structureless and dominant
incoherent contribution as the interaction strength is increased and the
injected density decreased, in a regime of parameters far away from the
mean-field limit. However, despite a partial dephasing, we observe that this
weak antilocalisation scenario prevails for finite interaction strengths,
opening the way for an experimental observation with $^{87}$Rb atoms.
