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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 luminousquasars (M_B ~ -23). The data are part of the GEMS imaging survey that provideshigh resolution optical images obtained with ACS in two bands (F606W andF850LP), sampling the rest-frame UV flux of the targets. To deblend the AGNimages into nuclear and resolved (host galaxy) components we use a PSFsubtraction technique that is strictly conservative with respect to the flux ofthe host galaxy. We resolve the host galaxies in both filter bands in 9 of the23 AGN, whereas the remaining 14 objects are considered non-detections, withupper limits of less than 5% of the nuclear flux. However, when we coadd theunresolved AGN images into a single high signal-to-noise composite image wefind again an unambiguously resolved host galaxy. The recovered host galaxieshave apparent magnitudes of 23.0<F606W<26.0 and 22.5<F850LP<24.5 withrest-frame UV colours in the range -0.2<(F606W-F850LP)_obs<2.3. The rest-frameabsolute magnitudes at 200 nm are -20.0<M_200nm<-22.2. The photometricproperties of the composite host are consistent with the individual resolvedhost galaxies. We find that the UV colors of all host galaxies aresubstantially bluer than expected from an old population of stars withformation redshift z=5, independent of the assumed metallicities. These UVcolours 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 bluestellar light in AGN hosts at lower redshifts. [abridged]

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