Mvied, different problems have been discussed in this thread. My post was to clarify whether the reported fix works for the one I reported, or is probably for a different one. Perhaps something in the way I wrote it made it seem like something else, but my intention was indeed to clarify, whether a problem was solved or not.
The problem with the URLs appearing at the top of the page, right before the adminbar, occurs even after disabling all other plugins. I could delete widgets only after disabling the plugin.
After erasing all the widget settings and trying out also the themes TwentyEleven and TwentyTen, I found that now, despite all widgets disabled and regardless of WordPress HTTPS being activated or not, there are three widgets visible: Search, Archives and Meta. If WordPress HTTPS is active, Archives displays only the archives links - the URL jumble mentioned before that with another theme went into the top of the page. If WordPress HTTPS is deactivated, Archives links appear and function correctly. With TwentyTen and TwentyEleven, these are shown in the secondary widget area and only if there are no other widgets active. With TwentyTen, after disabling other widgets, enabling them had no effect - only those three were shown.
Tried also on the live site on which the test site is based. Updating WordPress HTTPS caused the same URL jumble, which was solved by reverting back to 2.0.4. The theme in both cases is Photog with, you guessed it, lots of self-made modifications, none of which should cause the issue.
I then set up a different test site based on another live blog on the same server. There, roughly the same happened. Using ChocoTheme, no effects visible after updating to 3.0. With TwentyEleven and TwentyTen, the same. Troubles started when tried to alter or delete widgets. That didn't produce any effect while WordPress HTTPS was on. After deactivating it, deleting all widgets and then reactivating, TwentyTen and TwentyEleven now behave in the same way as with the other site. Activating ChocoTheme enabled two of the previously disabled widgets. After deactivating WordPress HTTPS and deleting those plugins the sidebar now shows these, presumably theme default, widgets:
Friends (which is Blogroll/Links renamed)
With WordPress HTTPS deactivated all those work as expected. With WordPress HTTPS activated, Archives creates the same URL jumble as in the other blog.