Context : Observational galaxy cluster studies at z > 1.5 probe the formation of the first massive M > 10 ^ { 14 } M _ { \sun } dark matter halos , the early thermal history of the hot ICM , and the emergence of the red-sequence population of quenched early-type galaxies . Aims : We present first results for the newly discovered X-ray luminous galaxy cluster XMMU J1007.4+1237 at z = 1.555 , detected and confirmed by the XMM- Newton Distant Cluster Project ( XDCP ) survey . Methods : We selected the system as a serendipitous weak extended X-ray source in XMM- Newton archival data and followed it up with two-band near-infrared imaging and deep optical spectroscopy . Results : We can establish XMMU J1007.4+1237 as a spectroscopically confirmed , massive , bona fide galaxy cluster with a bolometric X-ray luminosity of L ^ { \mathrm { bol } } _ { X, 500 } \simeq ( 2.1 \pm 0.4 ) \times 10 ^ { 44 } erg/s , a red galaxy population centered on the X-ray emission , and a central radio-loud brightest cluster galaxy . However , we see evidence for the first time that the massive end of the galaxy population and the cluster red-sequence are not yet fully in place . In particular , we find ongoing starburst activity for the third ranked galaxy close to the center and another slightly fainter object . Conclusions : At a lookback time of 9.4 Gyr , the cluster galaxy population appears to be caught in an important evolutionary phase , prior to full star-formation quenching and mass assembly in the core region . X-ray selection techniques are an efficient means of identifying and probing the most distant clusters without any prior assumptions about their galaxy content .