Selective Sync not working and cannot open files

Hi missingcow, thanks for reaching out to us about this. 

At first, note that selective sync and Smart Sync are two separate space-saving features. More specifically:

• Selective sync removes whole folders from your hard drive, while retaining them in your Dropbox account, and you can select to sync them back again at any point in time.  
• Smart Sync reduces the amount of space a file takes up by setting them to be online-only, keeping them available locally in your Dropbox folder. 

Having said that, could you let me know if you've tried converting content to online-only or selectively syncing folders in smaller batches?

As for the sync icons on your files, could you try the following steps in the exact order listed, and see if they do the trick?

• Click on the Dropbox icon in your system tray.
• Go to the gear in the Notifications panel.
• Select Preferences...
• Navigate to the Account tab.
• Click anywhere in the window under the tab.
• Hold down the 'Alt+H' keys.
• Click on the Fix Hardlinks button.
• You may be asked for your computer's login credentials, in which case, please enter them.

Wait a bit to see if Dropbox resumes syncing and, if you're still having trouble with this, repeat steps 1-6, but at step 7, click on the Fix Permissions button, instead. 

Let me know how it goes, thanks!