Problems with squid - restart solves for short time

Here is, what I get from console restarting squid:

 /etc/init.d/squid restart
Creating Squid swap directories...                                     [  OK  ]
Starting Squid Proxy Server...
2021/07/19 13:38:02| WARNING: BCP 177 violation. Detected non-functional IPv6 lo                                                                                                                               opback.                                                                [  OK  ]

What I can’t understand: When Squid is not working well, the clients can surf. But not elsewhere. And Updates for the linux-clients do not function. I found out, I can check the functionality of my squid-server if I start an online game. The viewer of this game points to FireStorm-Viewer Startpage when starting. If I don’t see this page, so the squid is not working well.

That really confuses me.