OpenVPN Error All TAP-Win32 adapters on this system are in use

This usually happens because the OpenVPN network adapter has been disabled or your already connected to the VPN. To resolve this

Windows XP:
OpenControl Panel > Network and Internet Connections Open Network Connections Right click the TAP-Win32 Adapter V9 and enable it.

Windows 7:
Open your Control Panel > Network and Internet > Network and Sharing Center > Change Adapter SettingsRight Click the adapter named TAP-Win32 Adapter V9 and enable it.

Alternative
You can re-install OpenVPN to re-create the adapter. Verify the old copy is not loaded in the system tray. 
You can also try running addtap.bat located in your OpenVPN directory.

If all else fails download the latest version of OpenVPN from http://openvpn.net/index.php/open-source/downloads.html 
Reboot your system and try to re-connect.

Alternatively try using the Liquid Viscosity

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