Answer
Credit Limit isn't actually stored on CUSTOMER_INFO — it lives on CUSTOMER_CREDIT_INFO (the Credit tab), so a MODIFY__ call against CUSTOMER_INFO_API silently does nothing useful to it and can surface this misleading concurrency error instead. Target CUSTOMER_CREDIT_INFO_API.MODIFY__ for that field instead, remembering that the Credit tab is company-specific (so Company needs to be in your key attributes) and that you need the current OBJID/OBJVERSION from CUSTOMER_CREDIT_INFO_CUST for the update to succeed.