I put some silly mistakes in the hostapd.conf.tmpl file which result in comment lines starting right at the end of configuration items:

vht_oper_chwidth=1# vht_capab: VHT capabilities (list of flags)
vht_capab=[...]# ieee80211n: Whether IEEE 802.11n (HT) is enabled

This needs to be fixed by clearing the minus (-) sign in the last jinja2 statement in the respective blocks.