Can't add a single folder to backup

I'm trying to get the Dropbox app to automatically back up a single file in on my computer. It's the file I keep all my work documents in. But for some reason, whenever I'm in Preferences to set up that backup, I can't mark that file to be backed up. I click on the box, but it remains unchecked. 

And when I go to click on the arrow beside that folder, to view the folder's contents (thinking maybe I can do a "select all" thing), the whole window goes completely blank for some reason. Like so: 

I'm at the end of my rope, here. I restarted my computer, logged out and back in, moved the Dropbox folder so that it's located in the same drive as the folder in question... it feels like I've tried it all. 
I even said screw it at one point and tried to have it back up my whole Documents folder. It was able to select them all, but of course I hit another error message, seemingly unrelated to the initial issue. 

Any help, friends?