I almost absolutely (I don't have an access to server's code after all) sure you have nothing to do with UserAgent.Seems like it does ;)
If you're getting 403 - most typical cause - you're visiting non-public page and logged in previously, so you got session cookie in your browser, while your HTTPClient lacks one.
so it's kind of common (ab?)use of such a response code.No, a page may not be rendered correctly for older browsers, so the 403 is correct. It is not abuse. Hence as a result, simply upgrade (acually spoof!) your useragent to something more recent.
6.5.3. 403 ForbiddenSource: RFC 7231
The 403 (Forbidden) status code indicates that the server understood
the request but refuses to authorize it. A server that wishes to
make public why the request has been forbidden can describe that
reason in the response payload (if any).
No, a page may not be rendered correctly for older browsers, so the 403 is correct.