That's where I initially (tried) to get it from after the prompt to update by the program itself failed too. I then tried the github versions archive which is what many of the third party links to Rufus use.
Perhaps it was just one of my specific shield settings that caused this but it is a bit odd that the problem only affected the last three versions of Rufus.
No matter and if others here have not had the same problem then its obviously not a general one.
EDIT
Just a small update which maybe of interest. I just did my weekly updates on another PC, very similar to the one with AVAST, but this uses another, lighter weight, AV.
I had a similar but not identical problem: it would not download the Rufus portable update when using Rufus' own updater prompt. But I could download it directly from the Rufus 'homepages' without any problems and the AV scan on that was clear too.
What this means I'm not sure but it does suggest that the Rufus installer/update download has a tag that may trigger some AV shields to block it ie. its not just AVAST.