Can't create Loyalty user using iVend API

Can't create Loyalty user using iVend API

Environment: 

iVend 6.5.5

Problem Description: 

iVend API Error

Scenario: 

Client is using iVend API to create loyalty users through below API as mentioned. As reported , it was working for more than a year but suddenly started throwing an exception. 
http://<IP Address> /iVendAPI/iVendAPI.svc/WebAPI/ReplicatePortalLoyaltyCustomerInformationToiVend/?siteId=1

Possible cause: 

Possible reason for same is that API is not working
iVend API working can be checked from iVend MC --> API Health check Screen and can be reached from the API from Window IIS screen.

Proposed Solution: 

Need to check iVend MC --> API Health check Screen and informed client to browse the API from Window IIS screen. Client can also browse the API from Window IIS screen. With these steps iVend API working can be checked.

Solution: 

Customer restarted the API service and issue was fixed.