Does Dropbox Auto Camera Upload work with iCloud Photo Library?
The answer seems to be "No". I tried various things around iCloud Photo Library and Dropbox Camera Uploads.  Here are some results.


Already since Sierra 10.12.2 (or slightly earlier?), Camera Uploads seems to only work if the iPhone is plugged in in an unlocked state.
After first enabling iCloud Photo Library, the "camera roll" contains a random selection of photos of different ages (most of them not even shot with the iPhone).  
The program "Image Capture" (comes with MacOS) which can be used to manage the Camera roll shows also these old photos.
Image capture can not delete any photos from the iPhone anymore, but it can still import these old photos and new photos taken.
Dropbox camera upload does not seem to do anything anymore after iCloud Photo library has been enabled.  It does not even display the little "Camera icon" right after connection.
I share pictures with my wife through Dropbox and I do this now by periodically exporting the unmodified originals from Photos on the Mac.