Contact Dropbox to issue you another key, then reenter that.
Rather than type the key just copy and paste.
This should help https://erpnext.org/docs/user/manual/en/setting-up/integrations/dropbox-backup
ERPNext writes those values to site_config.json.
This example thread gives details [V7] Dropbox Backup Error
In retrospect to save a copy of your site_config.json would have save you some effort to restore your DropBox settings…