Hello everyone,
I’ve seen a few topics around with the same or similar wording, but when I encountered this problem, none of the solutions proposed by Blizzard support or other users managed to fix anything (in fact some made the issue worse).
I believe I found a solution to the issue, at least for me personally. A few months back I had changed my router port forwarding settings to try to fix some net issues with my playstation. Those setting changes didn’t fix the issue I was having then, but I never undid them because I didn’t want to mess up the fix I had found for the playstation in case they did affect it.
I fully wiped and cleaned my computer trying to fix this update agent issue. After a complete wipe, I redownloaded the battle.net client installer and was greeted with the exact same error. I figured it had to be something external at that point and went and cleared out all of those port forwarding settings. Immediately after I tried the installer again and it quickly past the previous point it was stuck at.
I should also say that other computers on the same connection I’m using were playing/updating battle.net and world of warcraft just fine, so I have no idea why it was only preventing this on a single device but these port forwarding settings seem to be the (or at least a) culprit of this issue.
Hopefully this helps others facing the same issue and if a Blizzard support member sees this, could you look into this being a potential solution to the issue that you guys can advise other users to perform.
EDIT: If you are also having trouble with WoW (or other Blizz games) seemingly not being able to download (starts but never has a download speed, or gets stuck with a specific download speed but stops actually downloading), I also solved this problem by just updating the firewall configuration on my router. It doesn’t seem to matter what update, I’ve turned it off, turned it on, changed one thing, etc but as soon as I save any firewall changed the download starts up. I have no idea why this happens, how it works, or what to do to actually fix it (especially since my other computer on the same internet is still working just fine). I have CenturyLink as an ISP if that helps anyone finding this or helps Blizz look into the issue further.