That slash needs to go -- URL's are not escaped by slashes... so that's being treated as a literal. "can't change the name" just means "can't make it work on the web".
What are you using to generate the filename in the first place? Is it literally on the drive without the forward slash? If so, you need to get rid of that slash.
As a rule of thumb, it's ALWAYS a bad idea to use anything more than a..z, A..Z, 0..9, _, . or + in filenames without using the %XX escape codes.