MSTest - problem with private accessors

Question

I'm having a problem with MSTest where I'm getting hundreds of error messages along the following lines when I build my solution:

The best overloaded method match for 'ClassName_Accessor(Microsoft.VisualStudio.TestTools.UnitTesting.PrivateObject)' has some invalid arguments

Argument '1': cannot convert from 'SomeObject' to 'Microsoft.VisualStudio.TestTools.UnitTesting.PrivateObject'

or errors saying that accessors do not contains fields/methods that they should do.

I've been using MSTest in this solution for over a year without any problems and this problem started without me making any changes to the test project. Regenerating the private accessors doesn't seem to help. I've recently upgraded to SP1 but it's been running fine under SP1 for a few weeks before this happened.

Any ideas?