[en] We report the first results of an XMM-Newton monitoring campaign of the open cluster NGC 6231 in the Sco OB 1 association. This first paper focuses on the massive colliding-wind binary HD 152248, which is the brightest X-ray source of the cluster. The campaign, with a total duration of 180 ks, was split into six separate observations, following the orbital motion of HD 152248. The X-ray flux from this system presents a clear, asymmetric modulation with the phase and ranges from 0.73 to 1.18 x 10(-12) erg s(-1) cm(-2) in the 0.5-10.0 keV energy band. The maximum of the emission is reached slightly after apastron. The EPIC spectra are quite soft, and peak around 0.8-0.9 keV. We characterize their shape using several combinations Of MEKAL models and power-law spectra and we detect significant spectral variability in the 0.5-2.5 keV energy band. We also perform 2D hydrodynamical simulations using different sets of parameters that closely reproduce the physical and orbital configuration of the HD 152248 system at the time of the six XMM-Newton pointings. This allows a direct confrontation of the model predictions with the constraints deduced from the X-ray observations of the system. We show that the observed variation of the flux can be explained by a variation of the X-ray emission from the colliding-wind zone, diluted by the softer X-ray contribution of the two O-type stars of the system. Our simulations also reveal that the interaction region of HD 152248 should be highly unstable, giving rise to shells of dense gas that are separated by low-density regions. Finally, we perform a search for short-term variability in the light curves of the system and we show that trends are present within several of the 30-ks exposures of our campaign. Further, most of these trends are in good agreement with the orbital motion and provide a direct constraint on the first-order derivative of the flux. In the same context, we also search for long-range correlations in the X-ray data of the system, but we only marginally detect them in the high-energy tail of the signal.