Summary:

Issue with Document drop not working as it is not loading properly

Symptoms:

The Document drop icon is missing but the text remains.

Dragging a document onto the icon does nothing.

Cause:

A request to load some Javascript needed by the plugin was resulting in an error 500. This happened because a ServersideIncludes handler mapping was set up under IIS.

One of the handlers was setting IIS to attempt to execute any files with a JS extension on the server, rather than just serve it up to the client. Because it was client-side code, it failed when ran on the server, hence the error 500.

Resolution:

Using fiddler showed the failing request, and enabling detailed IIS logging made the cause of the error pop up in Fiddler.

Removing the SSI-JS handler resolved the issue.