What could be causing the 503 error?

Question

Nginx

There is a site www.all-smety.ru , it has authorization via HTTP, search engines and audit programs like Xenu and Screaming frog give a 503 error, despite the fact that the pages are available. Who can faced similar?
The site is on Bitrix, in the analytics section, transitions from search engines are not taken into account, everything is ok with this in the Yandex webmaster.

Answer 1

Elementary protection against parsers/scrapers. If in Xenu to deliver in 1 flow - that all normally fulfills. Most likely responds to IP + UserAgent + request rate.