I hadn’t been patching Windows since it was installed so it wasn’t a windows update that did it. It happened after an Avast program update about 2 versions ago. I did try and install the Windows creator update but that didn’t help either.
Do you have paid Avast support? I did try and log a ticket with them under the free version but they told me they won’t help unless I pay them. From reading the forums they appear to have done some firewall work for the paid version, there are people complaining about not being able to access router pages and the like but for them they can go an untick a box to resolve it, perhaps something has slipped into the free version as well.