(Dynamics AX) Failed to establish connection at Terminal Server

The "Failed to establish connection" in Dynamics AX is a common error seen and these are the common reasons:

1. AOS is down
2. AX configuration is incorrect
3. Firewall blocked the port used
4. Connection issue between client and AOS
5. User account is locked

But occasionally, my colleagues encountered this error at the Terminal Server (where all of us logon to), while there's no changes to anything (AX configuration, security, network, and etc), his/her AX suddenly doesn't works, can't connect to the AOS. Restart AOS or logon logoff from Terminal Server doesn't helps. Checked the 5 possible causes listed above, none of them is the cause.

I investigate further, AOS is running, port is not blocked, I logged on to the Terminal Server and start up AX client (all works fine, able to start up and forms opened OK). I asked a few other colleague to logon to the same Terminal Server and start up AX, all of them doesn't have the problem. So, what happens that only 2 of my colleagues unable to logon (Failed to establish connection)? The weird thing is the next day it's working fine for them, but on and off, it happens again (and to other users). Until today, I still doesn't know what could have cause this, but based on a random suspect, I do found the way to solve this - refresh group policy.

I was thinking, since it only happens to particular users, could it be profile or security changes? I then tried refresh the group policy, hooray, it works.

Here's what I do:

1. Open command prompt (Start > Run > cmd)
2. Type in "gpupdate /Force" and press "Enter"
   (it will refresh the group policy and prompt whether OK to logoff)
3. Press "Y" and logged-off
4. Login again and it is now working fine

If you know what could have cause this and why, do leave a message here, others could benefit from it.