Issue with Document drop not working as it is not loading properly
The Document drop icon is missing but the text remains.
Dragging a document onto the icon does nothing.
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.
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.