I'm running Kerio Connect Mail Server and am getting "Server is not responding. Please check your Internet connection" in Webmail. The Internet IS working - what's wrong?
Assuming that you mean you checked Internet to and from the actual Connect Server and found nothing amiss, there is a Connect setting in mailserver.cfg that you can tweak.
This was introduced in Kerio Connect 8.4.1. In that version, connections from the Kerio Connect Webmail Client are deliberately kept open for 10 minutes so that new messages can be pushed to the client sooner. However, a firewall, antivirus or reverse proxy (either on client or server) which intercepts HTTP or HTTPS connections could interfere with that connection and cause that error.
The variable is LongPollTimeout and it's default is 600 seconds (10 minutes). Stop the server, edit mailserver.cfg and change it to a lower value (100 seconds should be safe).