Error 206 is included in HTTP/1.1: Status Code Definitions.
- 206 - Partial Content
A status code of 206 is a response to a request for part of a document. This is used by advanced caching tools, when a user agent requests only a small part of a page, and just that section is returned.
Partial Content means that the user-agent is requesting the resource in "chunks" and not all at once
If user has a front-end suitelet that is marked as Available without Login and Execute as Admin is turned off, user will receive this error in the UI when page is accessed.
Similarly, a 206 error is thrown when nlapiRequestURL() API is invoked. To correct this, make sure that the front-end suitelet is accessible when accessed from a browser. Making Execute as admin=True should fix the problem.