Connection doesn't fall back to IPv4 when IPv6 is unreachable
Mumla may encounter a situation where a Mumble server has both AAAA and A DNS records, but is only reachable via IPv4 (not IPv6).
Most modern software (e.g. Firefox, Chromium, curl, OpenSSH) falls back to IPv4 in this case, either by trying both protocols in parallel or by trying IPv4 when IPv6 encounters an "unreachable" condition or timeout.
Unfortunately, Mumla doesn't seem to do either — the connection is just marked "offline" immediately.
This problem can occur when a Mumble server has an AAAA DNS record but the address is not configured on its network interface, or when there is an IPv6 firewall that's more restrictive than the IPv4 one (as in my case):
$ ping -4 mumble.fachschaften.org
PING bifrost.fachschaften.org (94.130.106.213) 56(84) bytes of data.
64 bytes from bifrost.fachschaften.org (94.130.106.213): icmp_seq=1 ttl=51 time=18.5 ms
64 bytes from bifrost.fachschaften.org (94.130.106.213): icmp_seq=2 ttl=51 time=17.9 ms
$ ping -6 mumble.fachschaften.org
PING mumble.fachschaften.org(bifrost.fachschaften.org (2a01:4f8:c0c:9463::1)) 56 data bytes
From fd20:1::1 (fd20:1::1) icmp_seq=1 Destination unreachable: Port unreachable
From fd20:1::1 (fd20:1::1) icmp_seq=2 Destination unreachable: Port unreachable
Here, the server's IPv6 range is blocked at the firewall to circumvent an upstream MTU problem that would otherwise drop packets silently.
Without that block, the connection attempts would get no response and run into a timeout, which should probably trigger a fallback to IPv4 as well.