Fixed for the next release, but not yet tried with a real alternate-start mod; waiting for the long test.
New characters normally play Vault 111 alone and then join, and alternate-start mods were untested with that. A character from an alternate start has the wrong identity until it is named, so it could not reliably host or join. The next release treats such a character like a Vault 111 one: it can host or join once it has a name. We have not been able to try it with a real alternate-start mod yet, so reports are welcome.
Change: pull request #176