How to fix – ERROR: All TAP-Windows adapters on the system are currently in use.

The error “All TAP-Windows adapters on this system are currently in use” suggests that there is a problem with the virtual Tun/Tap network adapter used by OpenVPN on Windows. To solve the issue, you need to re-enable the adapter or reinstall the driver.  

Solution 1: re-enable the adapter

Open Control Panel > Network and Internet > Network and Sharing Center > Change adapter settings.
Check description of network adapters to find the Tap-Windows Adapter.  
Right click on it > Disable.
Right click again > Enable.  

Solution 2: reinstall the Tap driver  

Check if it is already installed by opening C:\Program Files\TAP-Windows 
Right click on Uninstall and select Run as administrator 

Download the Tap driver from OpenVPN repository and install it (right click on it > Run as administrator)