Wish I'd seen this thread before I bought a new router. Mine was doing exactly the same. One thing I did notice was after doing a factory reset it seemed to be OK until I started to change the settings again. As soon as I tried to turn off Smart Setup it went straight to re-boot. A well, now I have a new router and a door stop.
I've just done the same as some others I have seen - hot-spotted and changed to Preview firmware. I worked. Here's some comments on the process I used.
I've now done the workaround and am back on the air.
For the record :
Firmware on chromecast that caused me the issue was : v1.21.74816
I then did the same to my other chromecast audios before turning the homehub back on and revisiting each of the chromecasts to put them back on the correct (homehub based) wifi connection.
Hope this helps someone else out.
I've had exactly the same issue. Yesterday morning I woke up to find my router had reset itself to its factory settings. Any time I attempted to change the SSID and password back to what it was before, the router would get itself in to a boot loop and eventually reset itself again.
I spent hours on the phone to BT support, believing it to be a router problem (as all the signs were suggesting a borked router). I was repeatedly told there was planned maintainence in my area and that's why my connection was dropping, and I should call back in 2 days time if I was still having issues. I protested that there was nothing wrong with the internet connection, and that it was the router itself that was problematic, but met fierce resistance. To be honest, the customer service experience was awful.
But then I looked on twitter, and found this - and it turns out it was my chromecast audio. Someone should tell the customer support people as it might save them (and their users) a lot of bother.
Similar to what others have posted, but if you can connect to your hub using an Ethernet cable you can then create a new wireless network independently of the hub and use this to update away from the broken firmware.
Wire computer to hub
Create new wireless network on computer
Setup Chromecast Audio on new wireless network
Wait until CCA gets new firmware
Connect CCA to bthub without issue.
For windows 10, see http://www.windowscentral.com/how-turn-your-windows-10-pc-wireless-hotspot
I know it can be done in other versions of windows, but I don't have links to hand.
Looks like both BT and Google are blaming the others hardware.