Error 520 on some regions and users, not on others

One of our websites, https://www.h2foz.com.br/, is presenting 520 errors on only one region of the country (Foz do Iguaçu region, Brazil).

Everyday, on about the last 7 days or so, visitors from this region are getting a high rate of 520 errors.

When I try to visit from other regions, like Sao Paulo, or even other countries, the error doesn’t occur. Also no errors are returned when using performance testing tools, like WebPageTest or GTMetrix, regardless of the chosen region.

What can we do?