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Kwallet broken


I don't know, what has happened, but suddenly Kwallet is no longer available. 
When I log in to plasma on my desktop, I get two windows, each asking me to 
enter a password - one for KMail and one for the Nextcloud-desktop-client. 
(I'm writing this on my laptop.)

There is this error message: "Couldn't open wallet: 
org.freedesktop.DBus.Error.NoReply; Message recipient disconnected 
disconnected from message bus without replying"

When I open the KWalletManager, the wallet is empty. 
What happened, and how to fix this?