A few users have reported issues trying to use the editor after upgrading their HE hub to platform version 2.4.3. This has now been fixed and is available as an update via HPM or by downloading the updated editor files from GitHub. The updates have been done so that they still work on HE platform versions prior to 2.4.3, so you can safely update Simple CSS Editor without needing to upgrade your HE hub.
I explain the issue in more detail on this earlier post:
This is the same issue discussed here:
I have also included the HTML file in the HPM package, but have not removed the download mechanism from the driver just yet. That will likely happen at some point soon.