That is strange - 404 normally means "file not found".
The commonest reason for this that I ahve seen is that you are using Calibre on Windows (which has a case independent file system) and the web side (drop box) is case dependent. Calibre2opds takes the name with the case specified in the Calbire metadata database so if the file is different in dropbox it might explain your problem. If it IS this problem, then the easiest fix is normally to change the metqdata for the author/title in Calibre (I normally add an 'X' on the end. Then having OK'ed that change it back again. This causes a rename sequence that should fix any case related issues.
If it is not that then I am at a bit of a loss as to the likely cause. If the Kindle browser shows you the URL it is trying it might give you a clue.
Note that there is now a
dedicated calibre2opds forum.