We report some results of optical observations of the transitional millisecond pulsar PSR J1023+0038, performed with the multi-channel panoramic photometer-polarimeter mounted on the 6-meter BTA telescope. The setup allowed us to register photons simultaneously in two channels (``red'' and ``blue'') with the effective temporal resolution of about 1 $\mu$s. We detected optical pulsations at the pulsar frequency with characteristics similar to the ones reported in other studies. In addition, during one of the nights we observed dramatic change in the pulse profile structure, mainly characterized by fast transition from double-peaked to single-peaked shape and back, and accompanied by flaring activity of the system. We analyse the results and discuss some aspects of their physical interpretation regarding the origin of the pulsing emission.