For some unknown reason, the security login form appears without being called on client desktop. My .Net project is a

SF project with security. the others projects in the solution are not SF ones. When the SF form is shown on the client desktop, if the form is not active for a while (approximately 10 minutes) the SF security login form appears. Very strange behavior! It happens to me while in dev using the debugger and I though it was related with a thread issue since i had several breakpoints and leaf my desktop in debug. In the Main() from program.cs file, the line (in 1.6 version)

//-- Stop the session monitoring before the application exits to remove the low-level event handlers

SessionLock.StopSessionMonitoring();

This line is not in the 1.6.1 version (in your examples). Is it a fix the my problem? If yes, please confirm. I post this issue as a knowledge transfer.

A quick answer will be greatly appreciated. 

You are experiencing the session locking and is by design.  There are a number of options you have here.  You can simply turn off the session monitoring by never calling the StartSessionMonitoring in the AppMain.vb (or program.cs) file.  You could also call StopSessionMonitoring() like you had suggested.  Or you can lengthen the time before timeout occurs in the Global preferences:

Finally, you can find everything relating to this in the help docs here:

Security Maintenance -> Global Preferences -> Global Preferences Properties

and

Security within the Application -> Session Locking

and

Defininf Security within the Application -> Programmatic Access -> SessionLock Class -> Methods

This is our goal for all of our customers.  Hope this helps.