This is a backwards compatibility issue. We will fix the problem and release a patch tomorrow. I'm sorry for the inconvenience.
I experience the same problem and get a 404 on: http://localhost:8080/file?fileID=&thumb=true&cachebust=1291284014871
It seemst hat the api changed to using fileID's, but that existing items have no such id or that the existing files need to be renamed.