初回のVagrant upがうまくいきません

Question

Windows10 のコマンドプロントで Vagrant up をしたいのですが、うまくいきません。初回です。
表示されているエラーメッセージは、最初、

An error occurred in the underlying SSH library that Vagrant uses.
The error message is shown below. In many cases, errors from this library are caused by ssh-agent issues. Try disabling your SSH agent or removing some keys and try again.

If the problem persists, please report a bug to the net-ssh project.

timeout during server version negotiating

だったのですが、ネットで調べたところ、

vagrant halt
vagrant up --provision

とすれば直るとあり、これを実行したところ、今度は、

Timed out while waiting for the machine to boot. This means that
Vagrant was unable to communicate with the guest machine within
the configured ("config.vm.boot_timeout" value) time period.

If you look above, you should be able to see the error(s) that
Vagrant had when attempting to connect to the machine. These errors
are usually good hints as to what may be wrong.

If you're using a custom box, make sure that networking is properly
working and you're able to connect to the machine. It is a common
problem that networking isn't setup properly in these boxes.
Verify that authentication configurations are also setup properly,
as well.

If the box appears to be booting properly, you may want to increase
the timeout ("config.vm.boot_timeout") value.

というエラーメッセージが出てしまいました。
どうすれば直るでしょうか？