Here’s the latest reply from Avast on this problem:
“Not our fault necessarily, but can be just effect of active network filtering (our network filtering) with combination with weakness in 3rd party network component (quite old (2015) Broadcom driver in this case). I’d suggest user to update this driver.”
See if you can find an updated driver here: https://www.broadcom.com/support/download-search
I did download Avast Driver Updater.
That found a few old drivers and one was the broadcom driver. Since updating all the old drivers, fingers crossed it hasn’t BSOD since.