Seems like a problem connecting to the vm to me not a passbolt problem.
From the machine running passbolt please check that:
nginx is running and php-fpm is also runing
nginx is listening on port 8081 (check it with netstat -pnuta for instance)
From that very same vm that runs passbolt you should be able to curl to the passbolt instance. You might need to pass a host header, for instance:
curl -Lvvk --header 'Host: 192.168.101.49' http://127.0.0.1