Context : Another outburst of the recurrent M 31 nova M31N 2008-12a was announced in late November 2013 .
Optical data suggest an unprecedentedly short recurrence time of approximately one year .

Aims : In this Letter we address the X-ray properties of M31N 2008-12a .

Methods : 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 .

Results : 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 \sim 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 .

Conclusions : 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 .