I get the message too. On this webpage it says the next:
http://jaka.kubje.org/projects/dropbox-uploader/
This means that the certificate of the Certification Authority (CA) that Dropbox uses for their SSL certificates is not installed on your system or PHP/cURL is not configured correctly to find it.
If you ARE the system administrator, try installing the CA certificates bundle to a system-wide location. If you use a package management system, this will ensure that they are kept up to date automatically. On Debian Linux for example, you can install the package ca-certificates.
If you are NOT the system administrator, you can download just the needed certificate and point DropboxUploader to it (before calling the upload() method):
$uploader->setCaCertificateFile("path/to/the/certificate.cer");
[ed. note: fourroses666 last edited this post 14 years, 8 months ago.]