Why does a certain site open every time from Ubuntu?

Question

linux

Problem situation: there is a network in which approximately 90% of computers are running Ubuntu OS. The rest are Mac OS and Windows. A few days ago, for some unknown reason, the site cian.ru stopped opening stably on all computers with OS Ubuntu. The site either opens immediately, or does not open at all with an error in the browser to check the Internet connection, or opens after hanging for 30-60 seconds. On Mac OS and Windows, the site opens without complaints immediately and without errors. No other sites were affected. The Internet works stably, without failures and is not heavily loaded. The problem site responds stably.
Sniffered traffic to the site. In case of successful opening of the site - no errors, a request is sent to the 80th port, the answer comes immediately and further until the page is fully opened. In case of unsuccessful opening, 2, 3 requests are sent almost simultaneously from different ports to port 80, and then several TCP Retransmissions occur and then either a successful response after 30-60 seconds, or the page does not open at all due to timeout.
All traffic goes through the server. To exclude its influence, it was turned off completely and traffic was directed through the router. Different DNS servers were also used. The problem has not been resolved.
When connecting a laptop with Ubuntu to another network, the site starts to open without problems. This results in a problem within a particular network.
Only one method remained untried - disconnect all computers from the network and connect them one by one until the problem reproduces again. This method is not a reliable way to identify the problem, since the problem itself is floating.
A temporary working solution has been found - the zenmate plugin is installed in browsers, which routes traffic through its servers.
Question: please help me understand what could change in the network, that a particular site stopped working on a particular operating system? What could be causing this kind of problem?

Answer 1

Try to lower MTU on the interface in this network.
ip link set eth0 mtu 1450 to begin with, reduce the number 1450 gradually.
eth0 - of course, you need to replace it with the name of the interface sticking out to the network.

Answer 2

mtu lowered to 1200

where did you launch it from? you need to track the entire route from your problematic network to your router. And of course, try to directly send an unfragmented packet of 1500 bytes from your router, and gradually lower it.
I don't feel like wangling, but it's still not clear on the L3 interfaces to your ISP, are you using gre tunnels by any chance? They tend to increase the packet size due to additional headers (GRE, IPSec, etc.).