There can be a number of issues preventing the connection – the two most common are different versions and firewalls. Please make sure that when you upgrade the server you upgrade all clients as well – to the same version. There are a number of ways to check the software versions – the first of them is to ensure that you used the installers with the same version number. If you don’t have the installers anymore you can check the client version number on the splash screen that pops up when you start the client. In case you don’t have a sufficient graphical display on the server, you can check the “server.log” of the server, located in the “logs” folder within the installation directory. Alternatively, feel free to send the logs to support@omixon.com for confirmation. When it comes to firewalls there are a number of test steps that can be executed to determine whether the firewall is preventing the connection – please see the “Client-server (TWIN 3.X.X AND EXPLORE 2.0.0) troubleshooting guide” for the detailed description.