  Another outburst of the recurrent M31 nova M31N 2008-12a was announced in
late November 2013. Optical data suggest an unprecedentedly short recurrence
time of approximately one year. In this Letter we address the X-ray properties
of M31N 2008-12a. We requested Swift monitoring observations shortly after the
optical discovery. We estimated source count rates and extracted X-ray spectra
from the resulting data. The corresponding ultraviolet (UV) data was also
analysed. M31N 2008-12a was clearly detected as a bright supersoft X-ray source
(SSS) only six days after the well-constrained optical discovery. It displayed
a short SSS phase of two weeks duration and an exceptionally hot X-ray spectrum
with an effective blackbody temperature of ~97 eV. During the SSS phase the
X-ray light curve displayed significant variability that might have been
accompanied by spectral variations. The very early X-ray variability was found
to be anti-correlated with simultaneous variations in the UV flux. The X-ray
properties of M31N 2008-12a coherently point towards a high-mass white dwarf in
the nova system. This object might be a promising Type Ia supernova progenitor.
We re-discovered additional X-ray detections of M31N 2008-12a that are
consistent with our data and increase the number of known nova outbursts to
seven. This nova is an exceptional object that merits further attention in the
future.

High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics