I had this problem, too; and a very frustrating one I had found it. At last, I solved it by running the COMMAND_PATTERN by hand, using the TEST_RUNNER for %m and the classpath for %p:
(my folder for the runner is called FitSharp)
FitSharp\Runner.exe fitnesse.fitserver.FitServer,FitSharp\fit.dll C:\users\me\myTestProject\bin\Debug\myTestProject.dll
This, finally, showed me what the Runner was unhappy about. In my case it was:
“attempt was made to load an assembly from a network location”
And to that problem, I quickly found the answer on Drew's 'Blog at
http://blogs.msdn.com/b/drew/archive/2009/12/24/xunit-and-td-net-fixing-the-attempt-was-made-to-load-an-assembly-from-a-network-location-problem.aspx:
*** Unblock fit.dll and fitsharp.dll! ***
Who knew?