TL;DR: The BLZBNTAGT00000841 error during an update can be caused by editing files or folders in the WoW install location, including the screenshots folder - so probably best to move screenshots elsewhere before doing anything with them.
I tried to open WoW this morning and was greeted with an update paused at 88%, and when I tried to resume it I got a BLZBNTAGT00000841 error. After a bit of log searching (thanks to Blizzard for making the Battle.net client and agent save lots of detailed log files), I found that it was erroring on trying to change the permissions on some screenshot files I’d just moved into subfolders.
Without going into excessive detail, I have a fairly strict file permission regime on my system, and by default Bnet/WoW aren’t allowed to write into folders I’ve created manually. Moving those subfolders out of the default Screenshots folder to another drive and restarting the Battle.net client let the update complete normally.
Why is the updater trying to change permissions on my screenshot files? Who knows.
For that matter, why does WoW save screenshots (and config files) to the application folder instead of the user folder? ¯\_(ツ)_/¯
But this may be a possible cause/solution for anyone who hits this error and has been doing screenshot sorting or any other sort of editing inside the WoW application folder.
(For the technically-minded: I found the permission errors listed at the end of the C:\ProgramData\Battle.net\Agent\Agent.8636\Logs\AgentErrors-*.log with the most recent timestamp. I presume the 8636 is a build ID which will probably change with future updates.)