when using the above combination DOpus hooks a floating horizontal toolbar not to the bottom edge of the screen (as intended by me), but maintains a distance equivalent to the (not present) taskbar.
I’m guessing that your machine has multiple monitors at different DPIs (or the system DPI has changed since the last login). If so, Windows has longstanding bugs where it can’t position docked toolbars correctly because it gets confused about where the taskbar is when working out where to place them.
Make sure none of the Compatibility settings in Windows are turned on for dopus.exe, as the DPI settings there can cause similar problems even with just one screen. None of those settings should be turned on for Opus 12.
No, non of that is the case. This seems strictly related to Start11 moving the taskbar to the top of the screen and there by confusing the mechanism for floating DOpus toobars without autohide on (!) attachted to the bottom of the of the screen.