Aha!
I now know which local device/s is/are causing this - a pair of Calex-branded Tuya-based RGBWW LED strip controllers. When they are powered on, they attempt to connect to each of the APs in turn, and cause the identical crash on both of them. I have absolutely no idea why; we've got other Tuya-based RGBWW lamps (not strips) in the house and they have no problems at all. Very unusual.