If the listing server is not available via IPv6 (like currently), Simutrans freezes when trying to open the server list.
However, when compiling with USE_IP4_ONLY, the freeze does not occur.
As far as I can tell, the server list is retrieved via a call to network_http_get, which in my case does a synchronous request via IPv6.
If the listing server is not reachable, the call will block until reaching the timeout, and only then will do a request via IPv4.