  We report on the variability of a multi-component broad absorption line (BAL)
system observed in the hyper-luminous quasar J1538+0855 at z=3.6. Observations
from SDSS, VLT, LBT and Subaru telescopes taken at five different epochs,
spanning 17 yr in the observed frame, are presented. We detect three (A, B, C)
CIV variable troughs exhibiting extreme velocities ($\sim$40,000-54,000 km
s$^{-1}$) similar to the ultra-fast outflows (UFOs) typically observed in the
X-ray spectra. The A component of the BAL UFO ($\rm v_{ufo}$ $\sim$0.17 c)
shows strength variations, while B ($\rm v_{ufo}$ $\sim$0.15 c) and C ($\rm
v_{ufo}$ $\sim$0.13 c) components show changes both in shape and strength,
appearing and disappearing at different epochs. In addition, during the last
observation on June 2021 the entire BAL system disappears. The variability
trends observed during the first two epochs (1.30 yr rest-frame) in the CIV,
SiIV, OVI and NV absorption spectral regions are the same for B and C troughs,
while the A component of the BAL varies independently. This suggests a change
in the ionization state of the absorbing gas for B and C components and
tangential motion for the A component, as causes of this temporal behavior.
Accordingly, it is possible to provide an upper limit for distance of the gas
responsible for the A component of $R\rm_{out}^{A}$$\le$58 pc, and in turn, a
kinetic power of $\dot{E}\rm_{K,ufo}$ $\le$5.2 $\times$ 10$^{44}$ erg
s$\rm^{-1}$. We also obtain $R\rm_{out}^{B,C}$ $\le$2.7 kpc for B and C
components, which implies an upper limit estimation of $\dot{E}\rm_{K,ufo}$
$\le$2.1$\times$10$^{46}$ erg s$\rm^{-1}$ and $\dot{E}\rm_{K,ufo}$
$\le$1.4$\times$10$^{46}$ erg s$\rm^{-1}$, respectively. Future spectral
monitoring with high-resolution instruments is mandatory to accurately
constrain physical properties of the BAL UFO discovered in the UV spectrum of
J1538+0855.
