Update: Problem solved. I've rebooted, and now it just works. I cannot reproduce the problem anymore myself.
I run KeePassX, then switch to another application. Now neither is the KeePassX window accessible by using Alt-Tab / task switcher, nor in the left-hand launcher icons. I have to manually move the other application windows out of the way to get back to the KeePassX window. I don't dare minimize it ...
I have the KeePassX icon added to the launcher, but it has no indicator for the open application window, and when I click it again, it just opens another KeePassX instance.
I would expect the KeePassX window(s) to be visible in the launcher and task switcher just like any other application windows.
Any idea what could be the problem?
Workarounds: Right-click on the KeePassX window title bar, and choose "Always On Top". Keeps the window in the foreground and I don't loose it. Or: Alt-Shift-Up also exposes the KeePassX window. (Found the latter on Task switcher not showing terminal windows)