Take a look to see what is happening on the server at night.Are you running AV scans? Any other application maint? I would say with what you are seeing there has to be something that is interfering with the backups.
What does it show in the Adamm.log? That is located in Program Files\Symantec\Backup Exec\Logs If you scroll all the way down towards the bottom it will be the most current information.
That doesn't make much sense. The 2 versions use the same serial numbers. I would suggest going into add and remove programs, repair the install and try entering the serial number there.