A too pale Black-headed Gull

Photos of an aberrant, adult Common Black-headed Gull / Lachmöwe (Chroicocephalus ridibundus) from Seewinkel, Austria. The head pattern and bill colour might pose a potential ID pitfall in regard to Slender-billed Gull / Dünnschnabelmöwe (Chroicocephalus genei). In the second photo it seems to me as if the regrowing, moulted primaries show a regular pattern, whereas the visible, long, outer primary appears all white.