Can not use DLL, even that it is in GAC and under AOT Reference

I have added a DLL in the AOT Reference  (AX2012 R2) that I have selected from GAC, but I get compile error just as it does not exist, the code intellisense does recognise it either.  It works fine if I add it in VS 2010.

Any suggestion what I can do (I've restarted AOS and Server)?