Boxcryptor Classic data isn't compatible with the "new" Boxcryptor.
That means, you must migrate first all your data before using it with Boxcryptor v2.
Here you can find the needed information:
To decrypt your existing data on an older macOS version, you should update Boxcryptor classic.
You will then have the possibility to decrypt and migrate your encrypted data.
You may find the download here:
https://www.boxcryptor.com/download/Boxcryptor_Installer.dmg (read only)
On Mac OS X 10.12 "Sierra" there are more Apple-restrictions.
To mount Boxcryptor Classic there, please use this version:
Please note, Boxcryptor will not run on Mac OS X 10.13 "High Sierra" and above.
The next step would be to create a Boxcryptor Account and download the latest release for your system.
You may find the download here: https://www.boxcryptor.com/download/
To migrate you files please carefully follow these instructions:
I hope this helps your transition to Boxcryptor.