  Observations of protoplanetary disks around very low-mass stars and brown
dwarfs remain challenging and little is known about their properties. The disk
around CIDA1 ($\sim$0.1-0.2$M_\odot$) is one of the very few known disks that
host a large cavity (20au radius in size) around a very low-mass star. We
present new ALMA observations at Band7 (0.9mm) and Band4 (2.1mm) of CIDA1 with
a resolution of $\sim 0.05''\times 0.034''$. These new ALMA observations reveal
a very bright and unresolved inner disk, a shallow spectral index of the dust
emission ($\sim2$), and a complex morphology of a ring located at 20au. We also
present X-Shooter (VLT) observations that confirm the high accretion rate of
CIDA1 of $\dot{M}_{\rm acc}$=1.4 $\times~10^{-8}M_\odot$/yr. This high value of
$\dot{M}_{\rm acc}$, the observed inner disk, and the large cavity of 20au
exclude models of photo-evaporation to explain the observed cavity. When
comparing these observations with models that combine planet-disk interaction,
dust evolution, and radiative transfer, we exclude planets more massive than
0.5$M_{\rm{Jup}}$ as the potential origin of the large cavity because with
these it is difficult to maintain a long-lived and bright inner disk. Even in
this planet mass regime, an additional physical process may be needed to stop
the particles from migrating inwards and to maintain a bright inner disk on
timescales of millions of years. Such mechanisms include a trap formed by a
very close-in extra planet or the inner edge of a dead zone. The low spectral
index of the disk around CIDA1 is difficult to explain and challenges our
current dust evolution models, in particular processes like fragmentation,
growth, and diffusion of particles inside pressure bumps.
