I've observed, if I'm logged in via ssh and started a lcl-qt5 application the error doesn't occur. The desktop environments and software release levels on both machines are more or less identical. So I've investigated the environment variables. In a ordinary local session the mentioned environment variables are set to these values:
KDE_FULL_SESSION=true
DESKTOP_SESSION=/usr/share/xsessions/plasma5
XDG_CURRENT_DESKTOP=KDE
In a remote session these variables are not set (why ever) and then the error not happens. If I unset these variables in a local session (all of them) the error is suppressed.
Unsetting these variables has some side effects. I'm using a german desktop and menu shortcuts with the control key like ctrl+c, ctrl+v, ctrl+x and so on are displayed as strg+c, strg+v, strg+x etc. Also the bitmaps on standard buttons (ok, cancel, close) are adapted to standard icons from the desktop theme. If the environment variables are unset, the menu shortcuts and button bitmaps are not changed to the local desktop settings.