3815150 - How do I solve error "503 Service Unavailable" for SAP Concur APIs?

I'm getting the error 503 when performing an API request. How do I solve it?

Integration - Client Web Services

Concur Expense all versions ; Concur Invoice all versions ; Concur Request all versions ; Concur Travel all versions

Client Web Services Support - Error 503 , KBA , CON-INT , Integrations , Problem

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