Unfortunately, BlackBerry devices do not trust SSL certificates issued by Let's Encrypt yet.
Therefore, you will need to add the IdenTrust DST Root CA X3 to your device in order to use the synchronization service (and this website).
To make it easier, I've uploaded the file to Dropbox so you can download it right from your device. Afterwards, just head to Settings -> Security and Privacy -> Certificates and add it. Dropbox link: https://www.dropbox.com/s/genqtg0pld623nd/identrust-dst-x3.pem?dl=1