I had the exact same issue updating my SD card over the weekend. I have Xfinity internet service and it turns out they have a new "Advanced Security" feature that blocked the maps download, marking it as a suspicious site. Once I turned off the "Advanced Security" the download worked without issue.
xFi (Comcast) Advanced Security turned out to be the culprit.
After reading theisen_4's post, I looked up Advanced Security online, and the information I found on Comcast's site talked about it in the context of Wi-Fi as a feature of the XFi App. Since I never installed that app and am connected to the internet via Ethernet, I mistakenly concluded that Advanced Security didn't apply to my setup.
Among the many, many things I've tried since my last post, I went to the Naviextras website and tried to download the maps from there. (To my surprise, it let me log in with the credentials I use in Mazda Toolbox.) When I clicked the Download button, a warning screen appeared in the browser advising me not to proceed. I took that advice, and made a note of the wording in the warning screen and the URL, which included "safebrowse.io".
Some googling led me to a forum post on Comcast's site explaining that the "safebrowse.io" site is produced by xFi Advanced Security. The post gave instructions for disabling Advanced Security on Comcast's website (not in the XFi app), which I implemented.
After doing that, I was able to download the map update with Mazda Toolbox. (Irritatingly slow, but it eventually completed.) It wasn't necessary to turn off Windows Defender virus protection or Windows firewall. I did the download on a Windows 7 computer that I had resurrected in one of my other attempts to try something else when I wasn't able to do the map update on my new Windows 10 computer. I'm pretty confident the map update will now work on the Windows 10 computer with xFi Advanced Security disabled. I guess I won't know until the next update becomes available, probably this summer.
Huge thanks to theisen_4. I doubt I would have found the solution without your input.
In case anyone needs it, here's the post from Comcast's forum with instructions for disabling Advanced Security:
1: Login to xfinity account.
2: Click the wifi icon on the tool bar. If you hold the mouse over it the tool tip will show Xfinity xFi. (wifi icon is to the left of the phone icon)
3: You should now be at
https://internet.xfinity.com/
4: Click on More...you will land here
https://internet.xfinity.com/more
[For me, there wasn't a More link to click. I just added /more to the URL. It was also necessary to page through some Comcast screens touting the product]
5: Click on My Services.
6: You will be at xFi Advanced Security...click Disable (or Enable).
7: You will be presented with a toggle button to select enable or disable.
It says to wait about 10 minutes for change to take effect.