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)