Problem solved!
Problem solved by reinstalling kwalletmanager (see https://wiki.ubuntuusers.de/KDE_Passwortspeicher/), removing "~/.local/share/kwalletd" and deacttivate kwalletmanager in usersettings (see https://wiki.ubuntuusers.de/KDE_Passwortspeicher/).

First: I have KDE Plasma 5.10.5 installed, my kernel-version is 4.13.0-21-generic and I'm running Kubuntu 17.10 on a 64bit system.

The Problem: Every time I start "Discover" I get a repeating error-message with the title "GPG-Backend for KWalletd - KDE-Service for Passwordmanager" and the GPG-Error-Message is: Decryption failed. (i get this error-message in german and i tried to guess, how it would look like in english)

I do not want to use KWallet. I tried to use KWallet for a time, and I guess I made a mistake while uninstalling it from my system, cause I remember me creating a Passwordcontainer called "schluessel" (german word for key).

My Question: Do you have any idea, how I can just make that error disappearing?

Thanks for your help,

Jan

P.S. Error Message in German: "GPG-Backend für KWalletd - KDE-Dienst für Passwortspeicher": Fehler beim Versuch, den GPG verschlüsselten Passwortspeicher schluessel zu entschlüsseln. Verwenden Sie eine Chipkarte, überprüfen Sie, ob sie richtig eingesteckt ist und versuchen es erneut. Die GPG-Fehlermeldung lautet: Entschlüsselung fehlgeschlagen.