Ok, I learned now, that I can either clear the directory /media/myusername frm ACL, or I can set special settings with "setfacl" or deinstall acl (worst idea).
But what d you suggest, should I do? I believe, there is some configuration file, where ACL are preset or preconfigured. Remember, I am using KDE, but from time to time also XFCE and LXDE. So this changes should not relate to KDE only, but globally. What I wonder, these ACLs are set to root by default (running Debian 12), but I would more expect, these ACLs should be related to the actual user. Is this maybe a bug? Or is this really wanted so. Best Hans