  The improved results on a direct search for a new X(16.7 MeV) boson which
could explain the anomalous excess of $e^+e^-$ pairs observed in the excited
8Be nucleus decays ("Berillium anomaly") are reported. Due to its coupling to
electrons, the X boson could be produced in the bremsstrahlung reaction e-Z ->
e-ZX by a high-energy beam of electrons incident on active target in the NA64
experiment at the CERN SPS and observed through its subsequent decay into
$e^+e^-$ pair. No evidence for such decays was found from the combined analysis
of the data samples with total statistics corresponding to 8.4\times 10^{10}
electrons on target collected in 2017 and 2018. This allows to set the new
limits on the $X$--$e^-$ coupling in the range 1.2 \times 10^{-4} < \epsilon_e
< 6.8 \times 10^{-4}, excluding part of the parameter space favored by the
Berillium anomaly. We also set new bounds on the mixing strength of photons
with dark photons (A') from non-observation of the decay $A' \to e^+e^-$ of the
bremsstrahlung A' with a mass below 24 MeV.
