Grabbit won't start in AEM 6.3 because JCRJobRepository uses "admin" session
Grabbit uses admin session for the JCR JobRepository implementation. In AEM 6.3, by default you cannot connect to JCR using admin session. It will throw this exception
Caused by: org.apache.sling.api.resource.LoginException: Bundle com.twcable.grabbit is NOT whitelisted

See this for more details : https://issues.apache.org/jira/browse/SLING-5135
If we want to continue to use admin for JCR JobRepository implementation, we will have to whitelist the package(s) that need it. Even better, we should come up with a way to not use admin for JCR JobRepository implementation at all. We can't really use the request session in this case, because the JobRepository connects to the JCR on bundle activation for example.
@jbornemann @jdigger
Had to make an additional config update to make sure grabbit worked(to overcome the not whitelisted issue)