  Context: The timescale of a microlensing event scales as a square root of a
lens mass. Therefore, long-lasting events are important candidates for massive
lenses, including black holes.
  Aims: Here we present the analysis of the Gaia18cbf microlensing event
reported by the Gaia Science Alerts system. It exhibited a long timescale and
features that are common for the annual microlensing parallax effect. We deduce
the parameters of the lens based on the derived best fitting model.
  Methods: We used photometric data collected by the Gaia satellite as well as
the follow-up data gathered by the ground-based observatories. We investigated
the range of microlensing models and used them to derive the most probable mass
and distance to the lens using a Galactic model as a prior. Using known
mass-brightness relation we determined how likely it is that the lens is a
main-sequence (MS) star.
  Results: This event is one of the longest ever detected, with the Einstein
timescale of $t_\mathrm{E}=491.41^{+128.31}_{-84.94}$ days for the best
solution and $t_\mathrm{E}=453.74^{+178.69}_{-105.74}$ days for the
second-best. Assuming Galaxy priors, this translates to the most probable lens
mass of $M_\mathrm{L} = 2.65^{+5.09}_{-1.48} M_\odot$ and $M_\mathrm{L} =
1.71^{+3.78}_{-1.06} M_\odot$, respectively.
  The limits on the blended light suggest that this event was most likely not
caused by a MS star, but rather by a dark remnant of stellar evolution.
