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You need to remove non-existing server references within K2 database.

Before attempting to remove anything be sure to backup your K2 DB (as usual / by default). In case you are not sure what are you doing just log a support ticket :)

Once you backed up your K2 DB perform the following steps:

Execute stored procedure Server.kClusterDown which removes references to node you removing from all the relevant tables, i.e.:

Server.ProcInst
Server.DestQueue
Server.Async
Server.IPCAsync
Server.ClusterMessage

You run this stored procedure specifying IDs (note: IDs not sever names) from Server.Server table as a parameters:

@ServerID = NEW SERVERID / ID OF THE SERVER WHICH STAYS IN YOUR FARM
@ClusterServerID = OLD SERVERID / ID OF THE SERVER WHICH YOU REMOVED FROM YOUR FARM
@ServerDownTimeout = 0

Once you executed this stored procedure remove references to the old server from the following tables:

SELECT * FROM Server.Server
SELECT * FROM HostServer.Server
-- License can be deleted from Workspace/Management GUI
SELECT * FROM HostServer.LicenseKey

After removing references to the old server from K2 database, restart K2 host server service to allow it to re-read changed configuration from the database.