can I trust Boxcryptor closed source software? Encryption of files for cloud hoster

That's a judgment call you'll have to make for yourself, but their technical overview has what I consider a huge red flag:

Which data is stored on the Boxcryptor Key Server

Private RSA key (encrypted with the user's password)

The fact that your private key is stored on their server, even if it's encrypted with your password, greatly weakens security. It means that, at best, the security is as strong as your password, rather than being as strong as the underlying encryption.

The storage of the encrypted private key on their servers mentioned by Mark is optional - you can choose to use the local account and distribute the keys yourself.