Solution: delete all spaces (and, probably, quotes) before and after the name of the interface in hostapd.conf:
nirn@arwen 11:18 /u/h/nirn> cat /etc/hostapd.conf
interface=wlan0
(...)
Lyrics:
Hostapd read config, but no trim spaces (probably for read strings such "wpa_pairwise=CCMP TKIP")...