OK, solved the problem.
The password missing was for Kopete. I just ran Kopete first, created a bogus Jabber Account, set a password for that and saved it with KWallet. After that, dragged and dropped my old kwallet.kwl into the manager and opened that second wallet. The old passwords, as well as the new ones, were now shown as passwords and not as binary data anymore.
I guess this works with other applications too, as my Konqueror passwords are shown the same way, but I can't guarantee that.
I hope I could help someone with this, I'm just glad I got my password back.
Good Night
Kolja