  [abridged] We present VLT/FORS1 spectroscopic follow-up observations and
HST/WFPC2 imaging of the system COSMOS 5921+0638, which exhibits quadruply
lensed images and a perfect Einstein ring. We investigate the nature of COSMOS
5921+0638 by studying its photometric, spectroscopic and physical properties.
We completed both an environmental analysis and detailed analytical and
grid-based mass modeling of the system. We measured the redshifts of the
lensing galaxy in COSMOS 5921+0638 (z_l=0.551+/-0.001) and 9 additional
galaxies in the field (5 of them at z~0.35). The redshift of the lensed source
was inferred by identifying a candidate Lya line at z_s=3.14+/-0.05. The mass
modeling reveals the requirement of a small external shear (gamma=0.038), which
is slightly larger than the lensing contribution expected by galaxy groups
along the line-of-sight (kappa_groups~0.01 and gamma_groups~0.005). The
estimated time-delays between the different images are of the order of hours to
half a week and the total magnification of the background source is mu~150. The
measured mass-to-light ratio of the lensing galaxy within the Einstein ring is
M/L_B~8.5+/-1.6. Our analysis indicates that the ring and point-like structures
in COSMOS 5921+0638 consist of a lensed high redshift galaxy hosting a low
luminosity AGN (LLAGN). Flux ratio anomalies observed in the lensed AGN images
are probably due to microlensing by stars in the lensing galaxy and/or a
combination of static phenomena. Because of its short time-delays and the
possibility of microlensing, COSMOS 5921+0638 is a promising laboratory for
future studies of LLAGNs.
