[robweber]

EDIT: On second thought, let me try to type in the path including the credentials manually. I'll report back...

That is most likely what you need to do. The example l posted is exactly how the string would appear when typing it out. Kodi is nice when adding sources through the gui in that it just asks you what you need to enter (ie, username/password) and then constructions this url string for you in the background. If you check your sources.xml and have a SMB source with a user/pass that is how it would look.

How can I check the log file on a device running mxlinux? I can't close the dialog box and get the URL because the next time I open backup the dropbox link changes.
Any ideas.

Your best bet is probably to use an SSH session via Putty or something. Just "tail" the log file and wait for the URL to pop up. If you can't do that is there a way to minimize Kodi while it's running and just find it on the filesystem? Sorry for all the trouble it takes to do this, Dropbox is designed for systems where you can pop open a browser window and authorize directly from that. Since Kodi doesn't have a browser this is the best we can do.