Running nunit-console.exe fails while GUI is also running

Question

I'm trying to run nunit-console.exe from a script and I get an 'Unable to locate fixture' error, but only when I have NUnit GUI also running. Once I close the NUnit GUI the script runs the tests just fine.

Here is nunit command being executed by the script;

"C:\Dev\Test1\Build\NUnit\nunit-console.exe" "C:\Dev\Test1\Tests\bin\Release\Test1.Tests.dll" /framework=v4.0.30319

It can even be a different version of the NUnit GUI that is running and it still interferes (ver 2.5.1 vs 2.5.5). It think it has something to do with the nunit-agent.exe as it runs (spawned by the nunit-console.exe process) when the GUI is not running, but it doesn't run when the nunit colsole runs while the GUI is also running.