Lync 2013 WAC - Either the network connection has been lost or the server is busy. please check your network connection.

In case you are publishing your WAC servers using TMG you might face the following,

Problem:
PowerPoint presentations not working and showing only (white screen) for External Users and a while it showing the following error message.
Either the network connection has been lost or the server is busy. please check your network connection.

Resolution:

1. In Forefront TMG, right-click the name of the new publishing rule and then click Properties.
2. In the Properties dialog box, on the To tab, select the option Forward the original host header instead of the actual one.
3. On the Traffic tab, click Filtering and then click Configure HTTP.
4. In the Configuring HTTP policy for rule dialog box, clear the Verify normalization check box and then click OK.
5. In the Properties dialog box, click OK.
6. In Forefront TMG, click Apply to enable the changes. When the Configuration Change Description dialog box appears, click Apply.