indeed, .bc file are not allowed on the Boxcryptor drive.
They should exist only at the location defined in Boxcryptor (like your Cloud or other location).
Of course, you can have .bc files on the Desktop, but to work with them you must copy them back to a defined location.
In case you want to manually decrypt something, drag out the corresponding file from the Boxcryptor drive (also, the one in plaintext) and not the .bc file from the Cloud.
Now, to fix your issue with the .bc files on the Desktop: move them back to a Cloud which is defined in Boxcryptor, wait for the sync, and they will appear in the Boxcryptor drive (under that location). Now you can open them.