Why Are 401 Errors Returned Inconsistently?

Error: 401
Message: Invalid credentials provided
Provider Message: Only one request may be made against a session at a time.

This error message can be thrown because NetSuite does not support making multiple requests at once using the same user credentials. If you have authenticated multiple instances of NetSuite with the same credentials or are making multiple simultaneous calls you need to either modify how your API calls are being made against NetSuite so that this timing issue does not occur, or pursue SuiteCloud Plus (SC+) licensing through NetSuite. There are different tiers of SC+ licensing, but they offer various levels of integration concurrency that permits concurrent requests from a given NetSuite account.