ReSharper recently switched the way it resolves references - it now uses msbuild instead of core Visual Studio to resolve some things. There are several things you could try:
- Install the latest ReSharper 8.2.1 version (RC was released today, April 25th, fixes tons of issues related to build errors)
- Go to ReSharper options, General, and at the bottom, uncheck the
Use msbuild to obtain project references
option. Then try rebuilding.
If the latter solves your problem, I suggest you open an issue on the ReSharper issue tracker, so they can fix it.
Hope that helps!