When dropbox://some_file is part of a jobsub_submit incantation, there are two phases: first upload some_file then call jobsub_tools to generate and submit JDF with some_file included in the transfer_input_files list. If the submit URL is a round robin DNS, some_file can end up on a different machine than where the jobsub_tools JDF generation and submission happens. This results in 'file not found' errors.
I can think of two fixes off the top of my head, 1) configure a web server on each backend server that can serve up dropbox locations for that machine, and transfer_input_files on the JDF generating server contains a URL to the dropbox file, or 2) allow jobsub_server to submit explicitly to the same server that the dropbox lives on, bypassing the DNS round robin server. Number 2) is currently forbidden by our SSL security settings.
There may be better fixes that haven't occurred to me yet