Not sure whether the upgrade to GW8 installed the right dictionary files?
Tip from Bob Jonkman :
When you find the .DC files open them in a text editor. The version
number is the first line in the file.
If all you have is “#VERSION=650″ (or no version line at all) you
won’t be able to upgrade the databases, although the new v8.0 agents
should work just fine with 6.5 databases.