Does this error only happen when trying to upload a Featured Image? Are you able to upload images in the Media Library?
It’s happening at the Media Library too.
Note: This issue it’s happening in all of my sites.
Sounds like maybe a file permissions issue that’s blocking the uploads from working.
I would check the permissions on your wp-content/uploads/ folder. I suspect it’s too restrictive. I believe it should be set to 646.
Full full detail on file permission settings in WordPress, see: http://codex.wordpress.org/Changing_File_Permissions
Now, with 646 value edited. Media Dashboard thumbnail it’s not showing.
Besides, a few hours ago, uploads were available and I didn’t change anything.
Note: One of the earlier upload attempts was uploaded 20 minutes later.
Weird behavior.
what’s your opinion on this?