http://bugs.winehq.org/show_bug.cgi?id=18290
--- Comment #6 from Lisa Denia eiffel56@gmail.com 2009-05-03 19:28:44 --- Confirming that the bug has something to do with the new ad.
The ad is a HTML page located under http://curseclient.curse.com/clientads-mpu.html and will be shown by the Curse Client in the addon list's bottom-right edge(CurseClient is an application that manages your game's addons). To achieve this, CurseClient embeds the Internet Explorer. The addon list is not shown until you register a game to manage.
A workaround is to install Internet Explorer 6 using winetricks. CurseClient will start up correctly and the ad will be shown as it should. Another workaround is to just use native shdocvw and shlwapi(as a dependency) from the Internet Explorer package. The ad will not be shown, theres just a blank square where it should be, but CurseClient works correctly. So I guess the bug lies somewhere in Wine's shdocvw ;).
A user in the official CurseClient forums reported that it works without tweaking if you have a paid premium account, which will disable the ad.