New Post: Verification of file signature failed for file

Ok, let's try with another certificate.

Open mmc => certificate => local computer.
delete "WSUS Publishers Self-signed" certificate from "Wsus" store, "Trusted Root Authorithies" store and "Trusted Publisher" store. Reboot your server.
Launch WPP and generate a new self-signed certificate. Import it in "Trusted Root Authorithies" store and "Trusted Publisher" store. Reboot your server.

Try to publish a new update.