Clearing the SPD cache helps – check out this useful tip from https://benprins.wordpress.com/2013/09/03/sharepoint-designer-clearing-the-cache/:
Solution
The solution to this problem is to clear the cache of SharePoint Designer.
1. If opened close SharePoint Designer
2. Open the folder %APPDATA%\Microsoft\Web Server Extensions\Cache
3. Delete the contents
4. Open the folder %USERPROFILE%\AppData\Local\Microsoft\WebsiteCache
5. Delete the contents
Out of many attempts to resolve the issue this one is the one that worked:
After I upgraded my computer from Windows 7 or 8.1 to Windows 10, I could not find any available wireless network in the list and the Network Connections Folder was empty. I noticed that the Network Connection icon in system tray just showed a yellow triangle with exclamation mark (!). Resetting network adapters configuration didn’t take any effect and I can find any way to fix the issue.
Very cool… Build custom file display handlers for Office 365:
http://dev.office.com/blogs/file-handler-add-ins-now-generally-available-in-office-365
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