I discovered a weird little problem, from which I think that other people had it too, but it was never resolved.
Before, when I switched the background image from a darker one to a light one, the behavior of the wingpanel was that the font color and the indicator icons changed from white to a dark color (close to black). But for some reason I don't know, this behavior suddenly changed: Now, if I switch to a light background image, the wingpanel switches from transparent to half transparent with black background, and the font and the indicator icons stay white.
It just changed this behavior without that I changed anything. Even more weird is: I just created a new user. And with the new user, the behavior is still the old one from before (so: when switching to a light background, the panel stays transparent but font and icons become dark).
So it seems, it must be something which changed on a user level. Actually, I liked the old behavior more. Any idea what the reason/issue could be?
Update: Unfortunately, this is happening again now after updating again. See my comment to the previously accepted answer below. Any ideas?