I found this googling around. I won't post link because I think the site where it originally was posted supports hacking activity, but here was the solution:
This error occurs shortly after launching the client and can be caused by a compatibility registry key for a library maplestory loads. It is fixed by removing the registry key.
The registry key is located at "HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers", and contains a value with the name consisting of the path to your maplestory.exe, and the Data being something similar to "$~ IgnoreFreeLibrary<nmconew.dll>". The dll can vary. Deleting the value fixes the issue.
We encountered some critical issues after going live that we had to bring the servers down for. Unfortunately, there is still no ETA. We appreciate your patience as we try to resolve these issues