I am trying to make callouts to salesforce from an external system using SOAP API. The API version is set to 37.0 but after a couple of days using the same implementation it fails, displaying the following error:
Error: INVALID_LOGIN: Invalid username, password, security token; or user locked out.
I checked all the options mentioned above and they all seemed fine. Later on I verified the login log history and I noticed that when the user failed to connect to the salesforce through the API, the displaying version was set to 36.0 while the previously working one was 37.0. This occured even though nothing was change in my instance.
Would you know what might trigger such an unexpected behaviour?