@elextr didn't know about the `FileChooserNative`, thanks. I tried it (in an quick'n'dirty attempt) but it looked just as the normal GTK3 file chooser dialog, so obviously I did it wrong.
@djhenderson: - we use prebuilt GTK, GDK and so on libraries from MSYS2 - as far as I know there are no debug symbols available
I still don't know what is causing the crash and how to easily debug it. Furthermore, I'm not willing to spend the necessary amount of time to entirely debug this, it probably would involve compiling the GTK manually to get debug symbols and more knowledge about the Win32 API regarding the file chooser dialog. I'm not a Windows user nor an expert on this OS.
Not sure what your exact use case is but copying the full path of a file is also possible the standard GTK3 file dialog by right clicking (no Shift necessary) a file or directory and choose "Copy location". Alternatively, the "Addons" plugin has an option to show a "Copy File Path" in Geany's Tools menu which does the same.
It might be better to remove the Windows native dialogs altogether instead of trying to fix their bad integration.