The error message: "An existing connection was forcibly closed by the remote host" usually indicates exactly what it is saying.
The connection was closed be something outside of the host (Mendix). Most often this happens when anything in your infrastructure thinks a time-out occurred. That could be IIS / nginx, but also your proxy server, firewall, basically anything between your pc and the server.
Most infrastructures that I have worked with have a time-out configured somewhere between 20-40 seconds, any request that takes longer than this time-out period is considered to have failed and will be cancelled.
It is likely that there is something during your login that takes long. This can be your onlogin microflow, but also your homepage microflow. If either take a long time that would at up to the login.
You should start by measuring the requests during login, you can easily do this with chrome developer tools, and try to identify what is taking this long, and which request will fail.