We are using LF Server 10.1.1 build 254

The accounts are LFDS Directory Accounts with Full Licenses assigned to them. They have also been added in the admin console to the LF Directory Accounts and Trusted for login.

I can repeat the issue on any machine, we currently do not have any Trusted devices registered. 

It was pretty easy to replicate the issue, across multiple LF repositories.

var repoReg = new RepositoryRegistration("LF-REPO-T", "Repo-T");
var username = "SDK-User-01";
var password = "********";

Session.Create(repoReg, username, password); // 1
Session.Create(repoReg, username, password); // 2
Session.Create(repoReg, username, password); // 3
Session.Create(repoReg, username, password); // 4
Session.Create(repoReg, username, password); // 5
Session.Create(repoReg, username, password); // 6
Session.Create(repoReg, username, password); // 7
Session.Create(repoReg, username, password); // 8
Session.Create(repoReg, "SDK-User-02", "********"); // This will fail to login. 

// Switch usernames and then SDK-User-02 will be able to connect, but 01 will not. 
// Repeat for any combination of LFDS Directory Users and the results will the same.

This is a known issue in the Laserfiche Server in 10.1, it's been fixed in 10.2.