NAV instantiates some temp folders and files based upon the combination of User Name and NAV Service Tier (NST). These cannot be shared.

So if you attempt to login from two different machines as the same Windows user on the same NST, the error following message will appear

An I/O exception occurred during the operation

You'll need to either

1. Create a second NST, which is easy, and connect to it
2. Make sure you don't share user logins across multiple machines