When looking in the module status, the GWAVA scan engine has failed, and will not start. The GWAVA log file shows this error:
2018-08-20 11:52:12 Collating scan object hierarchy
 2018-08-20 11:52:12 Object has circular
reference back to itself
 2018-08-20 11:52:12 Object id:35083 has
invalid hierarchy relationships and will be unlinked
GWAVA process won't start.
Check the Mail Filter Policy and check to make sure that the services/filters are not linked up twice to the same filter.
In one particular instance, the Named Executable Files were linked twice to the Virus Filter Group. This causes gwava engine to be sent into a loop and then crashes.
A double linked filter or service to a group, or another filter is the cause of this problem.