Odd, I reinstalled the add-on, since although it was in my grid, it wasn't listed as installed in preferences. It's working now.
Looks to be specific to the Dropbox add-on, I can drop to an e-mail, imgur, and Facebook. I installed the MAS version, but then replaced it with the downloaded one from the Aptonic site.
Recently updated, haven't used in awhile. When I try and drop files from Photos to dropbox I get the following Ruby stack trace.
[Launching task with command line: runner.rb task-92882]
Task description file task-92882:
ACTION: "/Users/mikelanghorst/Library/Application Support/Dropzone 3/Actions/Dropbox.dzbundle"
VARIABLE: "app_version" "3.5.4"
VARIABLE: "dragged_type" "files"
VARIABLE: "user_id" "email@example.com"
VARIABLE: "support_folder" "/Users/mikelanghorst/Library/Application Support/Dropzone 3"
Begin_Message: Copying DSC02069.jpg ...
/Applications/Dropzone 3.app/Contents/Actions/lib/rsync.rb:16:in `do_copy'
undefined method `each' for #<String:0x007fb7558155b0> (NoMethodError) from /Users/mikelanghorst/Library/Application Support/Dropzone 3/Actions/Dropbox.dzbundle/action.rb:69:in `dragged'
from /Applications/Dropzone 3.app/Contents/Actions/lib/runner.rb:71:in `call'
from /Applications/Dropzone 3.app/Contents/Actions/lib/runner.rb:71:in `<main>'
Running El Capitan