Some users encounter this bug with VirtualBox 6.1 on MacOS X:

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

This seems to be some VirtualBox 6.1 compatibility issue.

The quick solution is to completely uninstall and remove VirtualBox 6.1 and install version 5.2 instead, which is known to be a good version and works well.

Related: #96