When copying a new file to the DAV server it replies with "replace old one"?

[berndporr]

This is my DAV config (via ssl):

        dav_methods     PUT DELETE MKCOL COPY MOVE;
        dav_ext_methods   PROPFIND OPTIONS;
        create_full_put_path  on;
        dav_access    user:rw group:rw all:r;
        autoindex     on;
        auth_basic "restricted";
        listen [::]:443 ssl;
        listen 443 ssl;

if I copy a new file to the server I get the following strange confirmation dialogue. It's both under Linux and Windows:

If I confirm the file is successfully copied.
Not sure if that's a bug or has something to do how I've configured the server.