Recently we had deployed Desksite to a few users , however some of them when opening PDF would get a connection Dialog box opening up and clicking on Connect in the Server List would freeze the screen with 

“Not Responding” 

And the connection would stay disconnected, even though the user was connected fine to Desksite.

We made sure Adobe Protected Mode was disable and ran a Repair of Desksite.

In the End we had to remove and reinstall Desksite from Scratch to repair this