  In electroweak baryogenesis the baryon asymmetry of the universe is created
during the electroweak phase transition. The quantum transport equations
governing the dynamics of the plasma particles can be derived in the
vev-insertion approximation, which treats the vev-dependent part of the
particle masses as a perturbation. We calculate the next-to-leading order (NLO)
contribution to the CP-violating source term and CP-conserving relaxation rate,
corresponding to Feynman diagrams for the self-energies with four mass
insertions. We consider both a pair of Weyl fermions and a pair of complex
scalars, that scatter off the bubble wall. We find: (i) The NLO correction
becomes large for $\mathcal O(1)$ couplings. If only the Standard Model (SM)
Higgs obtains a vev during the phase transition, this implies the vev-insertion
approximation breaks down for top quarks. (ii) The resonant enhancement of the
source term and relaxation rate, that exists at leading order in the limit of
degenerate thermal masses for the fermions/scalars, persists at NLO.
