Comment 57 for bug 1748565

This Bug seems to affect even basic HCI functionality:
"hcitool inq" is never returning in my case. dmesg gives me said error message.
I have tried two different USB-Bluetooth Dongles:

[ ] usb 2-2: New USB device found, idVendor=04b4, idProduct=f901, bcdDevice= 1.12
[ ] usb 2-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ ] usb 2-2: Product: CYW20704A2
[ ] usb 2-2: Manufacturer: Cypress Semi
[ ] Bluetooth: hci0: last event is not cmd complete (0x0f)

[ ] usb 2-2: New USB device found, idVendor=0a12, idProduct=0001, bcdDevice=88.91
[ ] usb 2-2: New USB device strings: Mfr=0, Product=2, SerialNumber=0
[ ] usb 2-2: Product: CSR8510 A10
[ ] Bluetooth: hci0: last event is not cmd complete (0x0f)

Also btmon/hcidump just seem to freeze:

< HCI Command: Inquiry (0x01|0x0001) plen 5 #1 [hci0] 10.211282
        Access code: 0x9e8b33 (General Inquiry)
        Length: 10.24s (0x08)
        Num responses: 0
> HCI Event: Command Status (0x0f) plen 4 #2 [hci0] 10.314299
      Inquiry (0x01|0x0001) ncmd 1
        Status: Success (0x00)

The only way to recover from this state is to issue a "hciconfig hci0 reset".