尝试从资源字典访问图像资源时，多个NUnit测试类失败

I have a complex WPF project written in C# (.NET 4.0) which I wrote a couple of tests for (NUnit). Theses tests reside in different classes and as long as I run the tests for each class individually everything is fine. However, once I try to run all tests of all classes at once the tests of the first class succeed, but once the testrunner (Resharper or nunit-console) starts testing the remaining classes, all of them fail with the following stack trace.I have a complex WPF project written in C# (.NE