The file '/UrlRewrite.aspx' does not exist

Here's the back-story:
We have a big old app that's been recently migrated to .net 4 (from 3.5). While we're doing this we decide to move it to a new, more powerful server, because we expect a lot of growth in the near future. (server is running windows server 2008 r2)
With the .net 4 version on the new server we keep getting this error saying urlrewrite doesn't exist. Any ideas or aditional info I should give?
Here's what gets logged:
Message
The file '/UrlRewrite.aspx' does not exist.

Source
System.Web

Target site
System.Web.Compilation.BuildResult GetVPathBuildResultInternal(System.Web.VirtualPath, Boolean, Boolean, Boolean, Boolean, Boolean)

Stack trace
at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean e ...