Hi distrofit. I'm not a member of the development team, but I have a workaround sollution for you. I'm getting the same issue with the Show Desktop button. It is actually called "Minimise all windows", if that makes any difference, but I think that it considers the taskbar as a window or running application and so it minimizes it as well. You don't, however, have to restart openbox to restore your task bar and running applications.
In the right click menu you will find an entry for Desktops. There will be a sub-menu for, say, desktop 1 and desktop 2, and another for the list of running applications. Click on any application icon and you are back in your running session. The task bar will still be missing, although it will now show as an application called (panel) in the same sub-menu.
Alternatively, you can use the cycle windows option, i.e. Alt-Tab. On first press of alt-tab you'll see the list of running applications. After selecting one you will find that the application window is restored, but the task bar is still missing. On second press of alt-tab you will see a generic application icon in the list. That is your task bar. When you select it the task bar will be restored.
I think the "Minimize all windows" app needs to be tweaked to force exclude the panel (task bar) from the list of running applications. If I was a coder I would be doing it now ;)