If anyone else stumbles on this;
When I was trying to do an import in EPiServer I got a 404 Not Found error in the admin UI.
Blog posts suggested that this could be almost any kind of unhandled error.
I my case the problem was that the IIS maxAllowedContentLength was too small to allow the export file to be uploaded.
This web.config setting solved it:
The environment was IIS 7.5 on Windows server 2008.
Great, thanks for sharing!