Can't view some source files

While viewing a coverage report in NCoverExplorer I am unable to view some of the source files. A dialog box appears with the title "Resolve out of date file". But the file was changed 2 days ago, and since then I have recompiled the test dll and the production dll, and closed/reopened NCoverExplorer. The folder shown in the dialog box is the actual location of the file, and I can open other files in the same folder. Why does the app think that the file is out of date?

I'm using

NCoverExplorer 1.3.5.1921
NCover 1.5.5.0
NUnit 2.2.8
.NET 2.0.50727