I can't find examples (or think of one) where the genitive strong ending -en is used; that is, where the 1.) noun is singular and its gender is masculine or neuter, 2.) no determiner precedes the noun and 3.) the case is genitive.
An example like
Die Hoffnungen armer Menschen bleiben oft unerfüllt. (The hopes of poor people often remain unfulfilled)
uses the strong -er ending because Menschen is plural. Are instances of the genitive strong attributive adjective ending -en rare?