Image sampler keeps disappearing

So I have a large patch that is using an image sampler.

Sometimes when I open the GH file I get a warning like below and the image sampler is gone from the file. I add a new one, save the file, reopen the file and its gone again. But not always..

Anyone know about this or what might be causing it? 

The images are inside dropbox, but it wasnt a problem before. Using an image from a network resource doesnt work at all (it either takes really long to be responsive or crashes GH and Rhino).