When I browse to three specific sites (which happen to be run by the same provider), I get a 403 Forbidden Nginx error. However, the sites are up.
- I am able to access the sites when I use a proxy such as Hola or Tor to access the servers from a different network location.
- I am able to access the sites when I use two other ISPs than my usual.
- An admin of one of the sites reports that the problem does not occur with other users.
- The problem occurs in multiple browsers and devices.
- No other sites are blocked.
Apparently, something is blocking the connection with this specific ISP and this specific hosting service (run by a small web development company). The sites are not controversial, and in any case I have no reason to think that there is any sort of censor lurking on my connection.
What diagnostic tools can I use to figure out where the blockage is occuring?