Comment # 1 on bug 1129083 from Tim Irnich

Just as an additional information tidbit, there's a parallel thread on this
issue on
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/issues/123. 

They recommend to disable IPv6 connection checking with 
$ busctl --system set-property org.freedesktop.NetworkManager
/org/freedesktop/NetworkManager org.freedesktop.NetworkManager
ConnectivityCheckEnabled "b" 0

Which worked on my system.