  We have performed HST imaging of a sample of 23 high-redshift (1.8<z<2.75)
AGN, drawn from the COMBO-17 survey. The sample contains moderately luminous
quasars (M_B ~ -23). The data are part of the GEMS imaging survey that provides
high resolution optical images obtained with ACS in two bands (F606W and
F850LP), sampling the rest-frame UV flux of the targets. To deblend the AGN
images into nuclear and resolved (host galaxy) components we use a PSF
subtraction technique that is strictly conservative with respect to the flux of
the host galaxy. We resolve the host galaxies in both filter bands in 9 of the
23 AGN, whereas the remaining 14 objects are considered non-detections, with
upper limits of less than 5% of the nuclear flux. However, when we coadd the
unresolved AGN images into a single high signal-to-noise composite image we
find again an unambiguously resolved host galaxy. The recovered host galaxies
have apparent magnitudes of 23.0<F606W<26.0 and 22.5<F850LP<24.5 with
rest-frame UV colours in the range -0.2<(F606W-F850LP)_obs<2.3. The rest-frame
absolute magnitudes at 200 nm are -20.0<M_200nm<-22.2. The photometric
properties of the composite host are consistent with the individual resolved
host galaxies. We find that the UV colors of all host galaxies are
substantially bluer than expected from an old population of stars with
formation redshift z=5, independent of the assumed metallicities. These UV
colours and luminosities range up to the values found for Lyman-break galaxies
(LBGs) at z=3. Our results agree with the recent discovery of enhanced blue
stellar light in AGN hosts at lower redshifts. [abridged]
