error MT2001: Could not link assemblies.

Hi,
I have updated Mac's os to Mavericks and downloaded XCode 6.0.1. After that I have updated Xamarin to the newest version. But I cannot run my old projects on device. Every time i build i get the following error:
error MT2001: Could not link assemblies. Reason: Object reference not set to an instance of an object and that is it. My project uses PCLs with profile 158 and includes many libs installed via NuGet e.g: Microsoft.Threading.Tasks and System.Http.Net. I have tried everything. Rebooting system, cleaning project, creating new projects and coping files into them. Nothing works. I even tried to link all assemblies upon build - nothing. Also I have even tried installing the newest alpha version of xamarin and still same error. Please help me to resolve this problem.
Aro