I can also confirm that downgrading fixes the problem (used the 2:2.4-1.1 buster package for amd64 instead of 2:2.6-13).
Stopping NM and using a simple config file for wpa_supplicant only yields the same problem (pages of CTRL-EVENT-SCAN-FAILED ret=-22 ...). Using "02:00.0 Network controller: Broadcom Limited BCM43228 802.11a/b/g/n". [ 11.788080] wlan0: Broadcom BCM4359 802.11 Hybrid Wireless Controller 6.30.223.271 (r587334) Marc
