  (abridged version) We present a detailed spectroscopic and timing analysis of
X-ray observations of the bright radio-to-gamma-ray emitting pulsar PSR
B0656+14, which were obtained simultaneously with eROSITA and XMM-Newton during
the Calibration and Performance Verification phase of the
Spektrum-Roentgen-Gamma mission (SRG) for 100 ks. Using XMM-Newton and NICER we
firstly established an X-ray ephemeris for the time interval 2015 to 2020,
which connects all X-ray observations in this period without cycle count alias
and phase shifts. The mean eROSITA spectrum clearly reveals an absorption
feature originating from the star at 570 eV with a Gaussian sigma of about 70
eV, tentatively identified earlier in a long XMM-Newton observation
(Arumugasamy et al. 2018). A second absorption feature, described here as an
absorption edge, occurs at 260-265 eV. It could be of atmospheric or of
instrumental origin. These absorption features are superposed on various
emission components, phenomenologically described as the sum of hot (120 eV)
and cold (65 eV) blackbody components, both of photospheric origin, and a
power-law with photon index Gamma=2. The phase-resolved spectroscopy reveals
that the Gaussian absorption line at 570 eV is clearly present throughout ~60%
of the spin cycle. The visibility of the line strength coincides in phase with
the maximum flux of the hot blackbody. We also present three families of model
atmospheres: a magnetised atmosphere, a condensed surface, and a mixed model,
which were applied to the mean observed spectrum and whose continuum fit the
observed data well. The atmosphere model, however, predicts too short
distances. For the mixed model, the Gaussian absorption may be interpreted as
proton cyclotron absorption in a field as high as 10^14 G, which is
significantly higher than that derived from the moderate observed spin-down.
