  We present a detailed high-resolution weak-lensing (WL) study of SPT-CL
J2106-5844 at z=1.132, claimed to be the most massive system discovered at z >
1 in the South Pole Telescope Sunyaev-Zel'dovich (SPT-SZ) survey. Based on the
deep imaging data from the Advanced Camera for Surveys and Wide Field Camera 3
on-board the Hubble Space Telescope, we find that the cluster mass distribution
is asymmetric, composed of a main clump and a subclump ~640 kpc west thereof.
The central clump is further resolved into two smaller northwestern and
southeastern substructures separated by ~150 kpc. We show that this rather
complex mass distribution is more consistent with the cluster galaxy
distribution than a unimodal distribution as previously presented. The
northwestern substructure coincides with the BCG and X-ray peak while the
southeastern one agrees with the location of the number density peak. These
morphological features and the comparison with the X-ray emission suggest that
the cluster might be a merging system. We estimate the virial mass of the
cluster to be $M_{200c} = (10.4^{+3.3}_{-3.0}\pm1.0)~\times~10^{14}~M_{\odot}$,
where the second error bar is the systematic uncertainty. Our result confirms
that the cluster SPT-CL J2106-5844 is indeed the most massive cluster at z>1
known to date. We demonstrate the robustness of this mass estimate by
performing a number of tests with different assumptions on the centroids,
mass-concentration relations, and sample variance.
