It's because you were using Firefox when you accepted the certificate. Microsoft doesn't always play nice. Use Internet Explorer and install the certificate or use Firefox and go back to your site,
https://funambol.pcf.be:8443/funambol/ds, and export and save the certificate file. Then select Start → Settings → Control Panel → Internet Options → Content → Certificates → Trusted Root Certification Authorities and Import the certificate you just saved. Your Outlook sync should work.