Android VPN app shows "building configuration" and doesn't connect

If the VPN client for Android shows "building configuration" and immediately switches off, never attempting to connect, it means that there is a problem with the app's caching on your Android device. To solve it, you must delete its cache: 

Check in Android Settings > Apps & Notifications > VPN.AC > Storage > Clear Data and Clear Cache

Try again to connect after you cleared the cache. 

Other tutorials: