Running Server 2008 R2, IIS 7.5, if you get this error in the Application event log:

The Module DLL C:\Windows\System32\inetsrv\logcust.dll failed to load.  The data is the error.

It’s kind of an obscure situation. IIS is saying it could not load the logcust module, which is the Custom Logging feature. In my case, I was setting up a development server farm, and had installed this feature on one server. The shared configuration caused IIS to try loading this module on the other server.

Solution: install the Custom Logging feature. Server Manager -> IIS -> Role Services -> Add Role Services -> Grab the whole Health and Diagnostics category.