Hostapd not starting [x86_64, r11798, rt28xx]

Hallo,
hostapd is not starting automatically (timeout/ubus). It works when starting manually.

After 'wifi up':

Thu Dec 26 02:53:32 2019 daemon.notice netifd: radio0 (9078): command failed: Not supported (-95)
Thu Dec 26 02:54:02 2019 daemon.notice netifd: radio0 (9078): Command failed: Request timed out
Thu Dec 26 02:54:02 2019 daemon.notice netifd: radio0 (9078): Command failed: Not found
Thu Dec 26 02:54:02 2019 daemon.notice netifd: radio0 (9078): WARNING (wireless_add_process): executable path /usr/sbin/hostapd does not match process  path (/proc/exe)
Thu Dec 26 02:54:02 2019 daemon.notice netifd: radio0 (9078): Command failed: Invalid argument
Thu Dec 26 02:54:03 2019 user.notice root: ip link set dev wlan0 up

Note the 30 secs between line1 and line2. In this time period there is this process living:

$ ps aux |grep hostapd
root     10483  0.0  0.0    956   464 ?        S    03:01   0:00 ubus wait_for hostapd.phy0

After the 30 secs the process is gone (and no hostapd).

When i start hostapd manually wifi is working properly:

$ hostapd /var/run/hostapd-phy0.conf

/etc/config/wireless:

config wifi-device 'radio0'
	option type 'mac80211'
	option hwmode '11g'
	option path 'pci0000:00/0000:00:1d.0/usb2/2-1/2-1.1/2-1.1:1.0'
	option channel '13'
	option country 'DE'
	option noscan '1'
	option log_level '1'
	option htmode 'HT40'
	option txpower '10'

config wifi-iface 'default_radio0'
	option device 'radio0'
	option mode 'ap'
	option ssid 'xxx'
	option key 'xxx'
	option network 'wifi'
	option encryption 'psk2+ccmp'
	option disassoc_low_ack '0'

Any idea what's wrong?

OpenWrt SNAPSHOT, r11798-3e51cb7820
Linux OpenWrt 4.19.91 #0 SMP PREEMPT Wed Dec 25 10:59:16 2019 x86_64 GNU/Linux
Ralink rt2870 USB (rt2800usb/mac80211)