File transfer Android via USB stopped working

I do not know what changed, but copying files to the SD card of my Galaxy stopped working.
I can access the Galaxy, but when I drop files on it, I get this error message:
An error occurred copying xxxxx:
Unzulässige Funktion. (1).
(German for unautherized function).
It used to work with Directory Opus 11, then it did not work anymore (don´t know what changed).
Same error message with Directory Opus 12.

It does work with the windows explorer though....
Best regards,

Stephan