Network error when I'm using a proxy on my LAN

I use this site on multiple machines (laptop, mobile, desktop), from many browsers (for instance, on my android mobile I have a stock browser, a Firefox and a Chrome).
I'm using a squid proxy on my local network, to filter some unwanted sites (advertising, facebook etc.)
When I connect to hu.lichess.org (or en.lichess.org) from Firefox running on my mobile, it works.
Any other browser (whether chrome/stock browser on android, or any browser on the other machines) get an error message "Network error" on the top of the page, and can't show the number of players. As far as I see, every browsers, connecting via proxy, try to connect to socket.hu.lichess.org (or socket.en.lichess.org) except Firefox on the android, and get answer TCP DENIED/403.
If I disable the proxy, I can use lichess without any problems.
Could you help me, why is this difference?
Actually, the only difficulty caused by this error is, the site can not display the current number of online players. (Ther is a "?" in the place of that number)
As far as I see, every other functions work good.

ps: sorry for my horrible English... ;)

suddenly network errors also today. first on safari then to firefox. same results.

Yeah, happening here on chrome. Also had trouble on my ipad connecting to the lichess app.

As I see it could be because my firefox on android doesn't support websocket technology. Is it possible? If it is, then... lichess.org changed something in the background, and it uses websocket since today? Because I don't remember if I ever seen this error before today.

Using websockets via squid... khm... it's not a trivial thing as I see... :)

Although I can get connected, but it just loses connection seemingly at random.

#2, #3 and #5, are you all Squid/proxy users? I had no idea that stuff was so widespread o__O

#4 the symptoms do indeed suggest the inability to connect to WebSockets. I don't think any recent changes impacted that. FWIW Firefox on Android does support WebSockets, I have personally used the site from Fx on Android and can confirm it works. On the whole, I'm pretty sure it's a network issue rather than a browser/server issue.

I'm not big into computers, so this may be a stupid question, but what is a squid/proxy? I've heard of proxy, just not sure what it means.

Hellbal (#6), I have no idea what is it... I've changed nothing on my LAN. Yesterday I tried to reboot all computers, including the router, the proxy and the mobile, but it doesn't work, except FF on android.
An interesting thing in the squid log: FF doesn't try to access socket.hu.lichess.org, but the others do (with result TCP DENIED/503)

O.K., I've found a workaround, but I don't like it.
My squid allows using CONNECT method only to the standard SSL ports (actually only to port 443), but lichess tries to connect to other ports (As far as I remember, I've seen ports 7xxx at yesterday, and ports 9xxx today).
If I'd like to use this site via proxy, I need to allow all ports, but... khm... I don't want to do this.

Same problem here, would be nice if lichess could do something to make it work. Port 443 works for me, too.