we recently upgraded from Sitecore 7.2 initial version to the SP2 one. The upgrade went fine and everthing seemed to be working. However, we noticed that when trying to upload files through the (new) media library, the upload never finishes. I can see a 404 request to http://xx/api/sitecore/media/upload in the console, but I can't figure out what is missing. I have checked that I have the correct dll versions, and also use fresh config files for Sitecore.ItemWebApi.config.
The upgrade went fine and I have republished/reindexed everything.
Does anyone have any hints on what could be missing?