Nutanix One-Click Upgrade Error "Nutanix Cluster is under-replicated

One of the biggest operational pain points in 3 Tier architecture is maintenance. Regardless if we talking about Hypervisor upgrade, firmware update or any other maintenace jobs. At the end if one is lucky and all went well, then the mission is accomplished. The procedure of maintenance costs lot of time and money (and sometimes much nerves from the Admin who's  done the Job).
Nutanix has done their Job and gives an Option called "One-Click upgrade". You can do the maintenance Job with one-click (and with no downtime).
If you do an upgrade from NOS (with no Errors) and afterwards an upgrade from the Hypervisorsoftware, you will receive an error message with the following Information:

"Cluster is currently under-replicated"

This message occurs when the health-state from the NTNX-Cluster is on Level "warning" or "error" or the data resiliency state isn't "Ok".

Below is a procedure how to quickly check some states and solve the error:

Step #1 – check if you can see under replication from curator command line.

1. SSH Connection to a CVM
2. Log in with the appropriate credentials
3. curator_cli get_under_replication_info summary=true 

Step #2 - find ID for the last successful curator scan

1. curator_cli get_last_successful_scans 2>/dev/null

Step #3 - check where the curator master is running

1. curator_cli get_master_Location

Step #4 - log in to CVM with curator master is running and manually triger curator scan

1. SSH Connection to CVM with curator master
2. log in with the appropriate credentials
3. allssh curl http://localhost:2010/master/api/client/StartCuratorTask?task_type=2
4. Wait 5-10 minutes and check again if there is any unser replicated data (Step #1)