Known problem area on the E55. There is a recall, so check with MB or the dealer to see if your car has been addressed. Lots of information on this over at mbworld.org - there's a good size 211 AMG community there.

Solutions have ranged from a new gasket for the fuel pump/sending unit (cheap), to a new fuel pump/sending unit (not cheap), to a new fuel tank (expensive!).

Good luck.