The improved results on a direct search for a new
X
(
16.7
MeV
)
boson that could explain the anomalous excess of
e
+
e
−
pairs observed in the decays of the excited
8
Be
∗
nuclei (“Berillium or X17 anomaly”) are reported. Interestingly, new recent results in the nuclear transitions of another nucleus,
4
He
, seems to support this anomaly spurring the need for an independent measurement. If the
X
boson exists, it could be produced in the bremsstrahlung reaction
e\ud
−
Z
→
e
−
Z
X
by a high energy beam of electrons incident on the active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through its subsequent decay into
e
+
e
−
pairs. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to
8.4
×
10
10
electrons on target collected in 2017 and 2018. This allows one to set new limits on the
X
−
e
−
coupling in the range
1.2
×
10
−
4
≲
ε
e
≲
6.8
×
10
−
4
, excluding part of the parameter space favored by the X17 anomaly, and setting new bounds on the mixing strength of photons with dark photons (
A
′
) with a mass
≲
24
MeV
. For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling
ε
. This allowed a significant improvement in sensitivity despite a relatively modest increase in statistics
