fe-usergroup for new subscriptions is mandatory, even if tt_address is selected as storage.
tt_address does not have fe_groups but sys_categories (as well as direct-mail groups, AFAIR, if direct_mail is present).
Sorry. I was too dumb ...
Due to caching I did not get the tt_address version flexform!
But there is different problem: I see no settings for Admin-Confirmation with tt_address as storage.
fe-usergroup for new subscriptions is mandatory, even if tt_address is selected as storage. tt_address does not have fe_groups but sys_categories (as well as direct-mail groups, AFAIR, if direct_mail is present).