I had the same problem. Now i found it out.
In my router config the IPv6 support was activated. I turn it off and activate "allways use a native IPv4 connection". It seems that coc is no compatible with Ipv6.
At the same i turn the MTU value to 1490 bytes. But cant image that this is the prob.
Test it. Now its works. Before i use the Programm SecurityKiss to connect