Try adding at site_config.json the ip of your installation, that worked for me
"dropbox_secret_key": "YOUR SECRET KEY", "host_name": "http://YOUR HOST TO ACCESS ERPNEXT" }
Restart frappe and try to test the service, run:
bench restart bench --site site1.local --force trigger-scheduler-event daily_long
Take in mind you must grant access to dropbox in Setup>DropBox Backup>Allow Dropbox Backup. You will be redirect to ERPNext page showing you “Dropbox Access Granted”
Hope it helps,