SCOM agent not connecting. Events 20000,21016,20070

The OpsMgr Connector connected to MS.contoso.com, but the connection was closed immediately after authentication occurred.  The most likely cause of this error is that the agent is not authorized to communicate with the server, or the server has not received configuration.  Check the event log on the server for the presence of 20000 events, indicating that agents which are not approved are attempting to connect

OpsMgr was unable to set up a communications channel to MS.contoso.com and there are no failover hosts.  Communication will resume when MS.contoso.com is available and communication from this computer is allowed.

After all actions have failed like repair, reinstall, delete,approve again etc. The following steps should work. Use at you own risk and don't forget to backup your database.

Uninstall the agent from the computer completely first. Then do the following actions in sequence.

1. Delete pending agent if any

exec p_AgentPendingActionDeleteByAgentName ‘agentname.domain.com’

2. Delete
USE [OperationsManager]
UPDATE dbo.[BaseManagedEntity]
SET
[IsManaged] = 0,
[IsDeleted] = 1,
[LastModified] = getutcdate()
WHERE FullName like ‘%computername%’

3. Grooming

DECLARE @GroomingThresholdUTC datetime

SET @GroomingThresholdUTC = DATEADD(d,-2,GETUTCDATE())
UPDATE BaseManagedEntity
SET LastModified = @GroomingThresholdUTC
WHERE [IsDeleted] = 1
UPDATE Relationship
SET LastModified = @GroomingThresholdUTC
WHERE [IsDeleted] = 1
UPDATE TypedManagedEntity
SET LastModified = @GroomingThresholdUTC
WHERE [IsDeleted] = 1

EXEC p_DataPurging

4.  Groom all partition tables.

/*-------------------------------*/

declare @counter int  set @counter = 0  while @counter < 122

begin

exec p_PartitioningAndGrooming

set @counter = @counter + 1

print 'The counter is ' + cast(@counter as char)

end

/*-----------------------------*/