Thanks to everyone for their comments. I think that @rzamor1 is correct and that it is an access problem. Thanks too to @Charlie_Allingham for mentioning Windows Defender. It seems that my firewall software, which used to work independently, now sits on top of Windows Defender Firewall. I suspect that the problem is there. I have tried adding a new rule and will see how that goes.
I haven’t tied using the ‘Reset Treeshare’ option this time. As I mentioned above, I have used it before and written about it here. It does not affect the comms with Ancestry and is therefore unlikely to be the source of the problem. Briefly, after Treeshare has collected changes from Ancestry, it presents you with a screen listing (if no filters are used) all the people in your tree with an RM column and an Ancestry column. If you have a big tree, like mine with >66k people, there is a lot of data in this screen or available in the more detailed comparisons of individuals, and yet if all works well, it populates very quickly. This is because RM stores the Ancestry data previously used and only updates it with changes (inserts, amends, deletes) from Ancestry since its last use. Reset Treeshare deletes all the Ancestry data and rebuilds it from scratch. This would take >1hr in my tree if all was working well and might not finish at all if comms were continually being interrupted.