I've downloaded that newer version (OD+ 1.9i (build 537u)) via SDC as proposed - and I think that problem is still a bit newsworthy.
I've found out that -at least for me- it's got to do with the start menu's sub menus somehow. I usually use Ctrl+Escape to show the start menu. When I do that and NOT enter (open) any sub menus, then close the start menu pressing Escape (or mouse-clicking the desktop), the basic start menu shows fine when I click ObjectDocks start menu button as often as I like. Clicking "Run" or "Shut down" works fine the same way. But as soon as I enter any sub menu like "Programs" or "Settings" in either of them, Ctrl+Escape or ObjectDock's button, the latter "stops" working, meaning: it shows that error message mentioned above ("StartMenu helper file was injected but failed to take effect. Can not show the start menu at this time please try again."), but it actually opens the start menu at its default location, in the bottom left corner of the screen, instead of opening it at the ObjectDock's button's location. Edit1: This can't be used though, because the error message being opened automatically closes the start menu again./Edit1
As far as I can see, it would be able to reproduce that infinitely. After closing the error message I press Ctrl+Escape, then Escape and ObjectDocks start menu button works again (for one time - starting a program or whatever works).
Changing the start menu from the classic to the newer XP-looking thing or vice versa didn't solve the problem. OS is XP Professional with all current updates.
As aforesaid by Tofino, this isn't exactly a huge problem, since everything else seems to work and I'm too used to enter the start menu pressing Ctrl+Escape that I rarely click the dock's button, but it would still be nice to have it work these few times.
(I guess the separate start menu key found on some newer keyboards works the same way as Ctrl+Escape, but since I don't like it, my keyboard doesn't have such a key and I can't test it out.)
--harl