mja, thanks for the hint.
Just dl'd FileBox eXtender and while somehow limited in its functionality (from what I can see at first glance), it seems to work reliably.
And while it looks similar, it doesn't share the nasty habit I encountered with the bundled tools from PowerDesk and ExplorerPlus: those would, in case you chose a folder from the drop-down menu that had been renamed in the meantime, immediately dump all the stuff to be saved in the very first folder shown in the save box (especially "nice" if it's a save from Opera with dozens of small files at once, grrr). I had that happen quite often, therefore I removed them. In that case, FileBox eXtender shows an error message.
BTW: I haven't yet seen a "dialog extender" utility for Windows that would do what once was a favorite functionality of "Default Folder" for me on my Macs: You could click on any open folder in the background and the dialog box would open exactly that folder to save/open.
Does anyone know such a thing for Windows?