In case of an user who forgot his username the current version of FEU asks for
his password and some other properties marked for username lost inquiry in every
usergroup. But in most cases I presume a user who forgot his username also
forgot his password. So this does not work.

The lost username situation should be separated in two cases:

First case: The user should be asked to enter an e-mail address he stored in his
properties. This should include any e-mail properties in case two or more e-mail
props exist. If this e-mail is positively identified to belong to a
*distinctive* user, his username should be mailed to this adress. If this e-mail
belongs to more than one user we should just mail all usernames to this e-mail
address. If he shares access to this mailbox, not our problem.

Second case: The user has no e-mail address stored in his record. So now we
should ask him for some other details of this record, which are distinctive in
combination. There may be a lot of John Smith in the user database but with
different birthdays. To avoid conflicts like identical names as mentioned above
there should be an order to ask for properties to recover a lost username. So
John Smith must also enter his birthday and maybe his zip code too. But for
Israel Kamakawiwoʻole it is sufficient to enter his (sur-)name.