That’s what I suspected but it took a while to find out what the problem was.
To make a long story short, I found out that the permissions are set to root:root with the above mentioned mount command, regardless what they set before. I had success with following command: sudo mount -t cifs -o uid=mylocaluser,gid=mylocalgroup,username=testuser,password=secret //<myserver-IP>/share / /media/mnt
But there’s still an issue left. When I copy a file with drag & drop using the file manager Dolphin to the share and abort the process before it is finished, then the file is shown in the share but cannot be deleted or overridden?
I think it is problem of the OS, but maybe someone has an idea what’s going on there?