403 Syntax Error
Receiving the 403 Syntax Error error message while using AccountEdge Mobile, indicates the Dropbox application is not installed on your iPhone or iPad. Installation of the Dropbox application is a requirement in order to run AccountEdge Mobile.
To resolve this issue, simply install the Dropbox app, which is available from the App Store, on your iPhone or iPad. You do not need to uninstall and reinstall AccountEdge Mobile, and there will be no data loss.
Check the System Requirements for AEM page for additional AccountEdge Mobile Requirements.
If you need help with setting up AccountEdge Mobile in general please see this article.