The situation:

You have installed Sophos AV and Firewall with the Sophos Enterprise Manager.
Suddenly (maybe after months) the Autoupdate fails with a message like that:

"Meldung: Die Manifest-Datei für Produkt Sophos AutoUpdate war ungültig
Modul: ALUpdate"

"The manifest file for Sophos AutoUpdate is not valid" (something like that).

After searching the web, I found that interesting article:

Endpoint Security and Control: Abhilfe bei veralteten Sicherheitszertifikaten auf Endpoints
Fixing outdated security-certs on endpoints.

And THAT one did the trick for me:
Msiexec.exe /i "c:\program files\sophos\autoupdate\cache\sau\sophos autoupdate.msi" REINSTALLMODE=vdmus REINSTALL=ALL

That reinstalls Sophos out of the cache with some extra installer attributes.
After that, my client seems to work fine again.
 

Hope it does the trick for you,

Pete