Hi Jan,
Please offer us what platform WSUS you are using from your post i assume you meet the issue when your install update use WSUS, what you are trying to do
when you meet this error, this error can caused by the corrupt Scan update source, please try to re-sync your WSUS database from Microsoft update.
I found there have some third party article point out this error may
potential reason.
The certificate for the signer of the message is invalid or not found
http://chitrarasan.wordpress.com/2014/01/21/the-certificate-for-the-signer-of-the-message-is-invalid-or-not-found/
I’m glad to be of help to you!
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