We present results from a 20 ks XMM-Newton DDT observation of the radio-load quasar CFHQS J142952+544717 at z=6.18, whose extreme X-ray luminosity was recently revealed by the SRG/eROSITA telescope in the course of its first all-sky survey. The quasar has been confidently detected with a total of $sim 1400$ net counts in the 0.2-10 keV energy band (1.4 to 72 keV in the objects rest frame). Its measured spectrum is unusually soft and can be described by an absorbed power-law model with a photon index of $Gamma = 2.5pm0.2$. There are no signs of a high-energy cutoff or reflected component, with an 90 % upper limit on the fluorescence iron K$alpha$ equivalent width of $approx 290$ eV and the corresponding upper limit on the iron K-edge absorption depth of 0.6. We have detected, at the $> 95%$ confidence level, an excess absorption above the Galactic value, corresponding to a column density $N_H= 3pm2 times 10^{22}$ cm$^{-2}$ of material located at z=6.18. The intrinsic luminosity of CFHQS J142952+544717 in the 1.4 to 72 keV energy band is found to be $5.5_{-0.6}^{+0.8} times 10^{46}$ erg s$^{-1}$. We did not detect statistically significant flux changes between two SRG scans and the XMM-Newton observation, spanning over $sim 7.5$ months, implying that the quasar remained at this extremely high luminosity level for at least a month in its rest frame. We put forward the hypothesis that the extreme X-ray properties of CFHQS J142952+544717 are associated with inverse Compton scattering of cosmic microwave background photons (at z=6.18) in its relativistic jets.
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