I had exactly the same problem. Achat loaded up to 100 %, the music started to play, but before the avatar design screen or the possibility to login came up, I received a Windows error message, which wasn't too conclusive. (I'm running WindowsXP)
For me as well this happened suddenly, because I was able to use achat normally before.
I solved this by setting the compatibility mode of the files VersionUpdater.exe and achat.exe to “compatibility with Windows95”.
You can do this by right clicking on these files in Windows Explorer, select “Properties”, then select the compatibility tab and chosing compatibility with Windows95.
Things I tried before that didn't help at all were: uninstalling and reinstalling achat (even from a fresh download), updating DirectX, updating graphic card drivers (turned out I had the newest anyway) and curiously also setting the compatibility mode to Windows2000.
I hope this will help some people, it definitely helped me.