Trouble downloading large files. 'check internet connection' or 'connection interrupted"

I had the same problem.  Client is sending me large 4k video files from a shared link - about 50GB each.  They get to about 12GB and then fail every time. Each time I get the  "check internet connection" and then "site unavailable" errors. 

I am on ethernet, fiber connection. No problem downloading large files from Drive. I tried other browsers and the usual fixes. Restarting everything, clearing cache... The files playback fine inside the Dropbox media player. 

Next, I try to sync these files to a local folder using the Dropbox windows app.   Even weirder, when I connected the folder inside the dropbox app, it started syncing and one of the files downloaded properly, so I thought "great it works now!"/........nope.

After the first file downloaded fully, the other 2 files suddenly switched to "download complete status" - which they definitely were not. Tried reinstalling dropbox again, same result.  Instantly it says they are "complete" or they are stuck at the "waiting to sync" phase.  Dropbox has literally one job. I have editor access on these files, I have enough space on my local drive, and I've reinstalled Dropbox twice. 

The solution?  Completely shut down the computer, restart, and download the files.  Fixed.