I recently moved from Windows 7 to Windows 2008 server so I can run SharePoint on my physical machine and not through a VPC, so I've been trying to get everything re-installed on my system.  As part of that process, I tried re-establishing a connection back to one of client's corporate networks and their system prompted me to run JuniperSetupClientInstaller.exe.  Normally this runs, finishes, and you can connect to the VPN no problem.  This time, however, it failed.  Unfortunately, there were no error messages to let me know why - it just didn't work.

I've had success running application in "compatability mode" so I gave that a shot - same problem.  But during the installation I noticed that JuniperSetupClientInstaller.exe unpacks a number of files into a directory (you can see the exact location in the details of the installer) and then runs a DIFFERENT application - JuniperSetupClient.exe.  If you navigate to that directory, you will see a text file named JuniperSetupClient.log that contains information about the setup process.

In my case, I installed a SharePoint site on Port 3333 - which the Juniper software needs to communicate with the VPN.  There was a nice message in the log file saying the VPN software could not bind to port 3333 which quickly alerted me to the issue, and moving the site off that port number fixed the issue.  However, it would have been nice to had an error message of sorts because I spent a chunk of time futilely researching compatibility issues.