3804147 - ECP SCIM user CREATE fails with HTTP 500 and error "Enter a user name (Message No. 01279)" is shown in SLG1

When using Identity Provisioning to create users in an Employee Central Payroll, it fails with HTTP 500 when CREATE users. 

IPS provisioning log:

error=HTTP operation failed invoking https://myxxxxxx-sso.payroll.ondemand.com/sap/payroll/ecp/scim/v1/Users?sap-client=xxx with statusCode: 500, Response: {"schemas":["urn:ietf:params:scim:api:messages:2.0:Error"],"detail":"An error occured, check application log ; See log handle: ****","status":"500"}

SLG1 log:

• Enter a user name
• Message No. 01279

• SAP SuccessFactors Employee Central Payroll

Enter a user name, 01279, 500, An error occured, check application log, SCIM, IPS, ECP, user, CREATE, HRSFEC_ECP_CONFIG, ECP_SP48_20260112_1 , KBA , LOD-EC-ECP-PY , Payroll Integration EC to Employee Central Payroll , Problem