External monitor background changes on its own Mac OS X El Capitan

I use an external monitor with my macbook pro, with three screen spaces. Every time I open my mac, the same one of the external monitor backgrounds changes back to default. This also happens when I open change desktop background while my background is not the default, and as soon as I click on the folder where my preferred background is, it reverts to the default. I don't have 'change picture' checked.

I've tried using different backgrounds, some that are working fine on my other screens spaces, but it still does the same thing on the one screen. I tried deleting the space and replacing it with a new one, same result.