Cannot Export THIS File to THIS Folder

So, I have a folder with PSDs in it. No problem. I go to export (save for web) the PSDs to another folder within that folder, and I receive an error: "Could not complete this operation. The system cannot find the path specified." I'm using a plug-in to export, so I try manually exporting them: "Could not write the file. An unknown error has occurred." 

So, I tried troubleshooting. I tried exporting the files to another destination. Works fine. I tried exporting other files to this destination. Works fine. I tried moving (not copying) the folder, then creating a new folder with the same name in the previous destination and putting the files in it. Can't export to this folder either.

Users/Documents/Projects/Project1/Exports/JPG

--Cannot export from Project1 to JPG

Users/Documents/Projects/Project2/Exports/JPG

--Can export just fine with and without the plug-in

Users/Desktop/Project1/Exports/JPG

--Can export just fine

I've updated Photoshop and I've restarted it.

Apparently, I just can't export these files to this destination.  I know there are work arounds I could try, but at this point I need to know what's happening in case it happens again.