3129009 - Error Occurs While Calling C4C OData Service from External System

The below error occurs while gets below error message  performing a GET operation -

"Dear SAP Cloud for Customer User, Your system is currently unavailable. This is either due to maintenance that was scheduled within your agreed maintenance window. Then please try to access your system a gain after the maintenance window. Otherwise we apologize for the inconvenience. We are currently working to get your system back online as soon as possible."

 SAP Cloud for Customer

1. Go to POSTMAN/any API tool
2. In the Collection tab select GET for your specific service 
3. Click on Send
4. The system is displaying below message:

"Dear SAP Cloud for Customer User, Your system is currently unavailable. This is either due to maintenance that was scheduled within your agreed maintenance window. Then please try to access your system a gain after the maintenance window. Otherwise we apologize for the inconvenience. We are currently working to get your system back online as soon as possible."

1. Go to POSTMAN
2. In the Collection tab select GET for the given Collection and click Send
3. Go to the tab Headers and select Host <calculated when request is sent>
4. Click Send

Your system is currently unavailable ,  agreed maintenance window , c4c odata api. , KBA , LOD-CRM-INT-API , OData API (C4C Only) , Problem

SAP Cloud for Customer core applications all versions