@Leseratte_10: Not all dialogs remember their positions. The splash screen does not and the file dialogs are native dialogs from the system by default so calibre does not control their placement. The preferences dialog had a linux specific issue, fixed by
https://github.com/kovidgoyal/calibr...c166ac2cb5be92
The book reader by default does not remember its position/size. You need to enable it in the preferences of the reader (int he miscellaneous section). The Get books confirm/info dialog (and all such dialog really) are also not position remembering.
@JSWolf: If you disconnect a monitor, the size will only be remembered if the remaining monitor is the same size as the previous monitor the windo appeared in. Otherwise it will either be default size or it will try to fit the old geometry in the new screen as well as possible, making sure the entire window fits in the available space.