Periodically, some sites do not open with error code 451, why?

Question

Browsers

Quite often, when you open a site, it does not open, and the browser displays a page with an error 451, if you wait 2-4 minutes and refresh the page, then everything opens fine. At the same time, other sites are available at the same time and open without any problems. This problem is observed both in mozilla and in the Yandex browser, it was checked under win10 and ubuntu. What is the reason for this behaviour? The last time this was observed here:
lectures.uralbash.ru/999.additions/texteditors/vsc... I

refreshed the page in a couple of minutes and everything loaded.
Rostelecom provider.
The Internet comes from work over the air through NanoStation Loco M2, and at work such an error does not occur.

Answer 1

It seems that someone is MitM-ing you and Apache periodically crashes, on which scripts are running that process all traffic.
Try to go to the same address via TOR (do not forget to clear all caches), if the problem disappears, then the assumption is confirmed.

Answer 2

451 error - Not available for legal reasons. Most likely something on the side of the provider (well, or you live somewhere near the border, although this is unlikely to affect).