For some months I have had problems with the Dashboard in EPiServer giving me 404. I thought there was some problem in my configuration files, but apprently not.
The solution was that i was missing the UrlRoutingHandler in the IIS. You can add it by going to your web site in IIS and click Handler Mappings,
Then click Add Managed Handler.
Fill out the settings like this and click OK.
The handler is now added.
Then when you try to access the Dashboard again, everything is fine.