Hi!
Versions 1.3.x implemented a completely new mechanism for loading featured images, which did work for some users… but not for the vast majority. That’s one of the reasons for discontinuing that version, but it’s not the only one.
One of the main problems with that version is that, even though featured images are loaded from external servers, the “placeholder” in which they’re rendered is loaded from your own server. This shouldn’t be a big problem, if the placeholder were only loaded once, but unfortunately it’s loaded each time an external featured image has to be loaded.
As you can see, this results in a workload in your own WordPress server that doesn’t make sense: the idea of using external featured images is to reduce the data your server has to serve, but that version didn’t solve the issue.
I highly recommend you use one of the newest versions of the plugin and tweak your theme so that it becomes compatible with it.
Best,
David