Hello,
Please download this "unofficial" version of Boxcryptor Classic which should allow you to migrate your data even if you already upgraded to macOS Sierra:
https://dl.dropboxusercontent.com/u/1738091/Boxcryptor_Classic_v1.5.417.253_Installer.dmg
Older Boxcryptor Classic versions did not accept the password (even if it was correct) due to internal changes in macOS Sierra. See also my post here: https://forums.boxcryptor.com/topic/boxcryptor-classic-mac-os-x-1011-el-capitan/page/3#post-9196
Best regards,
Robert