Go into the Wonderland Online folder located in your hard drive or wherever you've installed it in, right click the aLogin.exe and choose properties.
Afterwards; choose the compatibility tab and make sure to tick the Compatibility mode to on and choose Windows 7, then in the settings part; Reduced color mode to be ticked on and choose 16-bit colors,tick disable display scaling on high DPI settings and run this program as administrator.
Make sure you don't run the program in full screen as you can change that in the main.exe's settings, where you are able to update your client.
Once done, open your game from the aLogin.exe and when it shows a black screen after you've clicked the ads and the mainpage quickviewer thing, press ctr+alt+del to put your computer in that mode, then either press esc or choose task manager after about 5 seconds. It should work fine.