Citrix clients can not connect to the citrix metaframe server

When trying to connect to the citrix server, the little connection window is displayed but ends with one of the following error:

Error in connection. The Citrix server is not available, please try again later.

or

Network or dialup problems are preventing communication with the Citrix server...

or

Terminal Server has ended the connection.

Here is a solution:

1. Delete this registry key on the client that can't connect: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSLicensing
2. Reboot the client computer
3. Retry to logon to the Citrix Server