Tor tries to connect over IPv6 in IPv4 networks with ClientAutoIPv6ORPort set
In #29641 (moved) we set
ClientAutoIPv6ORPort 1 to help users in IPv6-only networks. However, that leads to bootstrap errors, presumably because Tor is trying to use IPv6 in IPv4-only networks. See e.g.:
I've seen this myself as well when bootstrapping on a Windows machine at least. Just hitting the "Reconfigure" option and choosing "Connect" again "solved this issue.