This is similar to this issue:
https://forum.avast.com/index.php?topic=185359.msg1306111#msg1306111
Basically, Avast’s port-scanner protection has been disabling my access to my networked Brother printer. I have checked Avast’s logs, and this is indeed the feature that is causing this issue.
Short of disabling port-scanner protection, is there any way I can specify that connection attempts from the printer are to always be allowed?
What exact version of avast do you have installed ?
Update to the latest version (17.4.2294): https://forum.avast.com/index.php?topic=201729.0
I’m running that already:
Program version: 17.4.2294 (build 17.4.3482.0)
Virus definitions version: 170505-0
Perform a clean installation with the off-line installer for the latest version and check if the problem is solved.
Huh, I didn’t think that a clean install would help (you know the typical Microsoft “reinstall Windows” joke :-/ ), but after uninstalling it, and also running the separate uninstall executable, rebooting, reinstalling from scratch, it appears to be working now.
I also reinstalled only a minimal set of features (just the ones I use the most, including the firewall), so that might have done something.
In any case, it’s working again, so hooray. 
With avast it is often that for a (to me) unknown reason old settings or something like that get preserved when you update.
Well thanks for your suggestions!
You’re welcome.