Update: This issue has been reported to R&D and occurs when thousands of requests have been submitted [internal reference: SHARED-94723]. The workaround for the moment is to add an extra header to the request just after the function open()–For example, xmlHttp.open()–when the error occurs when executing a REST API call by the Run Script Workflow plugin and JScript is chosen as Language.
xmlHttp.setRequestHeader("Authorization", "Basic <Base64>");
Tip: The value <Base64> is the base64 string of username:password (colon incl.). In case of username:password, this value would be dXNlcm5hbWU6cGFzc3dvcmQ=.