  Afterglows of Gamma-Ray Bursts (GRBs) are simple in the most basic model, but
can show many complex features. The ultra-long duration GRB 111209A, one of the
longest GRBs ever detected, also has the best-monitored afterglow in this rare
class of GRBs. We want to address the question whether GRB 111209A was a
special event beyond its extreme duration alone, and whether it is a classical
GRB or another kind of high-energy transient. The afterglow may yield
significant clues. We present afterglow photometry obtained in seven bands with
the GROND imager as well as in further seven bands with the UVOT telescope
on-board the Neil Gehrels Swift Observatory. The light curve is analysed by
multi-band modelling and joint fitting with power-laws and broken power-laws,
and we use the contemporaneous GROND data to study the evolution of the
spectral energy distribution. We compare the optical afterglow to a large
ensemble we have analysed in earlier works, and especially to that of another
ultra-long event, GRB 130925A. We furthermore undertake a photometric study of
the host galaxy. We find a strong, chromatic rebrightening event at approx 0.8
days after the GRB, during which the spectral slope becomes redder. After this,
the light curve decays achromatically, with evidence for a break at about 9
days after the trigger. The afterglow luminosity is found to not be
exceptional. We find that a double-jet model is able to explain the chromatic
rebrightening. The afterglow features have been detected in other events and
are not unique. The duration aside, the GRB prompt emission and afterglow
parameters of GRB 111209A are in agreement with the known distributions for
these parameters. While the central engine of this event may differ from that
of classical GRBs, there are multiple lines of evidence pointing to GRB 111209A
resulting from the core-collapse of a massive star with a stripped envelope.
