What's the stacktrace given with the error. Please elaborate
Either you could allow anonymous users (see security settings of your model) or you could make sure that there is a user in your database with the right name and login password. Log in as MxAdmin and check if the user exists.
See this post from a couple of days ago: Need help connecting to Sandbox application
You need to go to project settings and add the mentioned microflow as the After Startup microflow