When I build (or rebuild) the solution with Visual Studio, there are no errors. However, ReSharper's Errors in Solution window shows errors such as "Cannot resolve symbol", "Cannot access private method here" or "Unexpected token".
Two files are affected. One is a .cs file and the other a .designer.cs file. They define the same class with partial. When I open the .designer.cs file in the Visual Studio editor, the errors disappear.
It appears that the problems where due to an encoding problem. The .designer.cs file contained sequences such as ä, ö and ü. I have changed them to the umlauts ä, ö and ü, respectively. That made the errors disappear.