Agents are requesting a policy using a blacklisted GUID.

Attempt to save messaging resource with GUID ... has been blocked by the Host Resource Blacklist.

ITMS 7.x, 8.x

Remove the GUID from the agent blacklist if the process was unable to get the new GUID out to the agent.

This sql query will delete from the AgentBlacklist machines that are actively trying to communicate with the server.

delete from AgentBlacklist 
where Guid in
  (
     select c.Guid
     from AgentBlacklist ab
     join Evt_NS_Client_Config_Request ccr on ccr.ResourceGuid = ab.Guid
     join vRM_Computer_Item c on c.Guid = ccr.ResourceGuid
     where ccr.StartTime > GETDATE () - 7
     group by c.Guid
     having COUNT (ccr.ResourceGuid) > 5
  )

If errors regarding the ResourceMerge table begin to appear in the logs after the above query is run this may need to be run as well.

delete from ResourceMerge 
where OldResourceGuid in
 (
  select vc.Guid
  from ResourceMerge rm
  join vRM_Computer_Item vc on vc.Guid = rm.OldResourceGuid
 )