This can happen for three reasons:
1. The update was corrupted upon download.2. You manually altered one of the update files (*.wyu).3. You rebuilt the update and replaced existing *.wyu (Update) files without also replacing the *.wys (Server) file
The Adler is a checksum that's calculated when the update is created and is added to the *.wys so that when updates are downloaded they are first verified to be valid before being installed.