Strange 404 behaviour

Hello here's the scenario:

This is happening for one IIS website of two. Windows 2008 R2:

This URL works fine: "http://website.com/category1/" (displays index.cfm)

This returns a 404 error: "http://website.com/category1/index.cfm"

Does anyopne have any ideas why this would happen? I'm pretty sure it's an IIS issue.

Thanks in advance