  In this work, we report the presence of rapid intra-night optical variations
in both -- flux and polarization of the blazar BL Lacertae during its
unprecedented 2020--2021 high state of brightness. The object showed
significant flux variability and some color changes, but no firmly detectable
time delays between the optical bands. The linear polarization was also highly
variable in both -- polarization degree and angle (EVPA). The object was
observed from several observatories throughout the world, covering in a total
of almost 300 hours during 66 nights. Based on our results, we suggest, that
the changing Doppler factor of an ensemble of independent emitting regions,
travelling along a curved jet that at some point happens to be closely aligned
with the line of sight can successfully reproduce our observations during this
outburst. This is one of the most extensive variability studies of the optical
polarization of a blazar on intra-night timescales.
