Issue

Lync Mobile Client won’t sign in.
Mobile client logs contain the error E_ConnectionError E2-2-1
Tests from testconnectivity.microsoft.com are failing.

 What we do know from testconnectivity.microsoft.com:

• the DNS record is resolving correctly
• the TCP port 443 is allowing the connection
• the SSL certificate seems to be OK

This means that we have made it onto the Reverse Proxy.

What appears to be failing is the jump from the Reverse Proxy to the Front End.

Following the Tell me more about this issue and how to resolve it link lands you on the following page

To be honest , that’s actually rather solid information from Microsoft..let us take a deeper look at this info

A Network Error Occurred while Communicating with Remote Host (Front End)

• The server is down.
• The specified port is not listening for TCP connections.
• A firewall, proxy server, or hardware device is blocking the connection to your server.
• The network is congested.

Resolution

So what the underlying issue here boils down to is that for some reason the traffic is not reaching the Front End Server from the Reverse Proxy.

Of course this could be that the Reverse Proxy is incorrectly configured.

From my experience, the most common causes in a new deployment are:-

• Reverse Proxy cant resolve the host name for the Front End
• Ports are blocked (from Reverse Proxy to Front End port 4443)
• Forgot to port forward 443 to 4443 (or 80 to 8080) on the Reverse Proxy

TIP

The E2-2 errors usually refer to a connectivity issue. E2-2-1 generally means that the client couldn’t reach the destination.

Conclusion

When you get redirected to a hyperlink do take the time to consider the common causes. Who knows, you might even find the answer..