What happened here exactly? Did I configure something wrong? Had a (bogus - but that's not the point) radar detection event on a Unifi 6 Lite configured for HE80 on channel 100. The problem is that it did not come back to life by itself after the mandatory 30 minutes silence. Instead it failed like this:
Jun 27 13:40:01 u6-2 hostapd phy1-ap0: DFS-RADAR-DETECTED freq=5500 ht_enabled=0 chan_offset=0 chan_width=3 cf1=5530 cf2=0
Jun 27 13:40:01 u6-2 hostapd dfs_downgrade_bandwidth: no DFS channels left, waiting for NOP to finish
Jun 27 13:40:01 u6-2 hostapd phy1-ap0: AP-DISABLED
Jun 27 13:40:01 u6-2 hostapd phy1-ap0: AP-STA-DISCONNECTED 78:12:34:56:e5:f7
Jun 27 13:40:01 u6-2 hostapd phy1-ap0: AP-STA-DISCONNECTED 84:78:9a:bc:b2:ec
Jun 27 13:40:01 u6-2 hostapd 20/40 MHz: center segment 0 (=106) and center freq 1 (=5510) not in sync
Jun 27 13:40:01 u6-2 hostapd nl80211: deinit ifname=phy1-ap0 disabled_11b_rates=0
Jun 27 13:40:01 u6-2 netifd Network device 'phy1-ap0' link is down
Jun 27 13:40:02 u6-2 hostapd phy1-ap0: interface state ENABLED->DISABLED
Jun 27 13:40:02 u6-2 hostapd phy1-ap0: interface state DISABLED->COUNTRY_UPDATE
Jun 27 13:40:02 u6-2 hostapd phy1-ap0: interface state COUNTRY_UPDATE->HT_SCAN
Jun 27 13:40:02 u6-2 hostapd could not get valid channel
Jun 27 13:40:02 u6-2 hostapd phy1-ap0: interface state HT_SCAN->DFS
Jun 27 14:10:34 u6-2 hostapd phy1-ap0: DFS-NOP-FINISHED freq=5500 ht_enabled=0 chan_offset=0 chan_width=0 cf1=5500 cf2=0
Jun 27 14:10:34 u6-2 hostapd could not get valid channel
Jun 27 14:10:34 u6-2 hostapd phy1-ap0: interface state DFS->DFS
Jun 27 14:10:34 u6-2 hostapd phy1-ap0: DFS-NOP-FINISHED freq=5520 ht_enabled=0 chan_offset=0 chan_width=0 cf1=5520 cf2=0
Jun 27 14:10:34 u6-2 hostapd phy1-ap0: interface state DFS->DFS
Jun 27 14:10:34 u6-2 hostapd phy1-ap0: DFS-CAC-START freq=5500 chan=100 sec_chan=1, width=0, seg0=106, seg1=0, cac_time=60s
Jun 27 14:10:34 u6-2 hostapd 20/40 MHz: center segment 0 (=106) and center freq 1 (=5510) not in sync
Jun 27 14:10:34 u6-2 hostapd Can't set freq params
Jun 27 14:10:34 u6-2 hostapd DFS start_dfs_cac() failed, -1
Jun 27 14:10:34 u6-2 hostapd phy1-ap0: DFS-NOP-FINISHED freq=5540 ht_enabled=0 chan_offset=0 chan_width=0 cf1=5540 cf2=0
Jun 27 14:10:34 u6-2 hostapd phy1-ap0: interface state DFS->DFS
Jun 27 14:10:34 u6-2 hostapd phy1-ap0: DFS-CAC-START freq=5500 chan=100 sec_chan=1, width=0, seg0=106, seg1=0, cac_time=60s
Jun 27 14:10:34 u6-2 hostapd 20/40 MHz: center segment 0 (=106) and center freq 1 (=5510) not in sync
Jun 27 14:10:34 u6-2 hostapd Can't set freq params
Jun 27 14:10:34 u6-2 hostapd DFS start_dfs_cac() failed, -1
Jun 27 14:10:34 u6-2 hostapd phy1-ap0: DFS-NOP-FINISHED freq=5560 ht_enabled=0 chan_offset=0 chan_width=0 cf1=5560 cf2=0
Jun 27 14:10:34 u6-2 hostapd phy1-ap0: interface state DFS->DFS
Jun 27 14:10:34 u6-2 hostapd phy1-ap0: DFS-CAC-START freq=5500 chan=100 sec_chan=1, width=0, seg0=106, seg1=0, cac_time=60s
Jun 27 14:10:34 u6-2 hostapd 20/40 MHz: center segment 0 (=106) and center freq 1 (=5510) not in sync
Jun 27 14:10:34 u6-2 hostapd Can't set freq params
Jun 27 14:10:34 u6-2 hostapd DFS start_dfs_cac() failed, -1
and it was stuck there until I manually restarted the interface.