The following are some general tips that may be useful before performing any more detailed investigations:
Try pinging the node.
Try connecting to the Couchbase Server Web Console on the node.
Try to use telnet to connect to the various ports that Couchbase Server uses.
Try reloading the web page.
Check firewall settings (if any) on the node. Make sure there isn't a firewall between you and the node. On a Windows system, for example, the Windows firewall might be blocking the ports (Control Panel > Windows Firewall).
Make sure that the documented ports are open between nodes and make sure the data operation ports are available to clients.
Check your browser's security settings.
Check any other security software installed on your system, such as antivirus programs.
Click Generate Diagnostic Report on the Log page to obtain a snapshot of your system's configuration and log information for deeper analysis. This information can be sent to Couchbase Technical Support to diagnose issues.