Hi Adrian,
Unfortunately, suppressing this issue won't help here, as when tests results are reported out of order, NCrunch has no way to make sense of the results. The error invalidates the entire test run. You need to find out why NUnit is running your tests in parallel.
Make sure you are using the latest version of NUnit. Search your entire codebase for ParallelizableAttribute and ParallelScope. See
here for a detailed guide on the configuration options available for this under NUnit, and make sure you don't have this configured. To my present knowledge, NUnit parallelisation is the only thing that can cause the error you've encountered.
If you're able to isolate the issue in a sample solution you can share with me, I'll be happy to trace the engine through it to understand what is going wrong.