При возникновении данной ошибки в OpenVPN:

All TAP-Windows adapters on this system are currently in use.

Варианты решения проблемы:

1. Рекомендуется в «Сетевых подкючениях» переподключить «Подключение по локальной сети», у которого в свойствах установлен «TAP-Windows Adapter V9».
2. Удалить из C:\Program Files\TAP-Windows адаптер, запустив «Uninstall.exe» и установить его заново.

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In english:

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)
If you don’t have access to openvpn.org but you have our software, open our software location (where it was unzipped) and find the driver in the data\Tap\ subfolder.