after digging a little deeper into this issue all seems to be a local problem here with my machine. i have a software called WinSize2. it recognizes a certain programs window size and position and restores size and position in case the program does not do this by itself. somehow i have accidently configured WinSize2 to maximize the spacedesk server about windows. after deleting this setting in WinSize2 everything is fine.
issue can be closed. sorry for that.