Created Unassigned: Error: Some update files aren't signed correctly. Error code: (0x800b0109) [382]

Our parent company recently changed our WSUS server to be a downstream replica of their server. Prior to this I was using WSUS Package Publisher without any issues. After they changed the WSUS configuration, I could no longer revise or create new updates. I've just changed our on site WSUS server back to synchronise directly with Microsoft rather than another update server. I can now revise and create new packages using WSUS Package Publisher. However, I have a problem installing them on the client machines. I've created a superseding update for a piece of software we use and have approved it for installation. When I search for new updates on a client PC, it finds the new update, downloads it, but fails to install. It comes up each time with the following error message:

Some update files aren't signed correctly.

Error code: (0x800b0109)

Any ideas how I fix this?

Many thanks,
Mark