OpenVPN Error: All TAP-Windows adapters on this 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 
Reboot the system (not required, but recommended)
Download the Tap driver from OpenVPN repository and install it (right click on it > Run as administrator)

Other tutorials: