If you are unable to access Webmail/Admin when using Kaspersky Antivirus/Internet Security, it may be Kaspersky which is blocking the site.

One user has reported that you can solve this by the following steps: - Go to the Kaspersky Web Anti-virus Section - Add the server URL & Webmail port (default 5108) to the Trusted URLs Section (eg 'http://192.168.0.1:5108)