Boxcryptor v1.x and v2 are not compatible to each other. Files encrypted with v1.x (which we renamed to "Classic") cannot be decrypted with v2 and vice versa. However, v2 has an importer to convert v1.x files to v2.
As ez2517 pointed out, accounts are new in Boxcryptor v2. If you'd like to upgrade to v2, you should create a new account (either offline or online) and import your "old" v1.x Boxcryptor Folder.
If you are happy with v1.x and do not want to upgrade to v2, you can download Boxcryptor Classic (v1.6) which we will continue to support in the future.