Hello,
There was a change to either Shopify or Dropbox in early February that made it so that uploading images from a Dropbox URL would no longer work. A number of people have encountered this and I’ve written some more details in this post.
To summarize, it no longer works and unfortunately it seems like your only options are to either store the images elsewhere or to use a third-party app (like our Ablestar Bulk Product Editor) to import the image URLs. Mentioning the issue with Shopify support might also help raise awareness for the problem.
Best,
Daniel