Not sure if this has been remedied in SS > 3.0.3 (the version I found this bug), but images cannot be added via URL if Development mode is enabled. I have not found anyone else having this issue, so it may be unique to my installation, but thought I would share.

To reproduce the bug:
- Select "Insert Media" button on Content WISYWIG
- Choose "From the web" tab
- Enter an image URL and click the Plus (+) button
- Get white screen (first time I got a server error screen)

Removing the following lines from my _config allows adding an image this way to work:

Director::set_environment_type("dev");
ini_set('error_reporting', E_ALL);
ini_set('display_errors', 1);
error_reporting(-1);