So, I dug out my old laptop and found that it was very low on space. The last time I used it was before Dropbox started using the Save hard drive space feature, so it had a ton of local files that ought to be online only.
I did a stupid thing and tried to trick Dropbox. I dug up a external hard drive with plenty of room, quit Dropbox, moved the files to the external HDD, started up Dropbox. I expected it to copy files back into the folder as online only, which it did. Except…
The next time I logged in, it threw an error about missing a folder (Desktop) in the external hard drive that wasn’t plugged in. The path was to the Dropbox folder.
I searched the Dropbox configuration and help without finding anything helpful. I searched on startup issues and eventually found something helpful.
DANGER! DANGER, WILL ROBINSON!
So, before I state what I found, here is the thing. This involves the registry which a novice user should leave alone. At least make a backup of the registry with File > Export.
I screwed up a computer well enough I had to go into safe mode and fix my mistake once. Another time, I ended up thinking I knew the mistake but didn’t and had to re-install the operating system from scratch. That was back in the day when I had the media (CD-ROM) to do that kind of stuff. Most of the time I export the registry and don’t have to go to such extremes, but it was also like 20 years ago when I was less cautious and brasher about using a sledgehammer to drive a nail. And if I screwed it up, I would spend hours fixing it when today I might sheepishly take it to someone else for help. (Or go to Linux.)
Anyway, the “User Shell Folders” at
Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders has the entries that caused this behavior. The Desktop one was the one actively causing the issue.