Failing Batch Unit Tests in ReSharper on accessing config file

I am having problems with my unit tests (NUnit) run with ReSharper. I have no problem executing each test individually, nor all tests in the project. Problems start when I execute all tests in the solution. Same tests are running perfectly fine executed with NUnit Adapter and VS Unit Test Explorer. I've managed to established, that tests are failing on accessing config file. Changing methods responsible to read values to hard coded and I am having no problems, but that is not a solution to the problem. I could create separate class, just to read config values, but I thought it is perfectly fine to get them through the method within the class (mvc controller). I have been trying to find the answer how to cure those problems, as it sounds a bit silly to avoid using config file - that's the sole purpose of that file in the first place.