Lance Ahlberg was running into an Argument Exception when using NUnit. See how he fixed it:

I have been getting an argument exception with details “Absolute path information required”, this exception was happening when I was running Tests with Nunit.

After almost pulling my hair out the problem was found, Environment variables....

Nunit caches the assemblies that are being tested in the temp directory, and uses the TEMP environment variable.  Of course my TEMP environment variable was a relative path.

So the moral is, if you see Absolute path information is required take a look at your environment variables and ensure that they are absolute.