Hello Matthias,
Ah ok, I think there's a misunderstanding here. I was talking about Boxcryptor 2.x while I assume you're using Boxcryptor Classic.
If you want to restore a single file from a backed up Boxcryptor Classic Folder, you have to at least also restore the .encfs6.xml file located on the root of the Boxcryptor Classic Folder and place it together with the restored encrypted file into a new folder. Then you should be able to open this folder in Boxcryptor Classic and access the encrypted file.
Example:
-- RestoredFolder.bc
---- .encfs6.xml
---- RestoredFile.txt
Best regards,
Robert