I am currently upgrading from 10.2 to 10.3 and as expected there are always some issues with recompiling addins and upgrading the the visual studio version.
There is now a known issue for which there is a workaround described here.
My question is what is the function of the ESRI.ArcGIS.AddIns.targets file? What does it do and why does renaming the new ESRI.ArcGIS.AddIns.11.targets to the former ESRI.ArcGIS.AddIns.targets work? Is it what packages the actual dll for use with ArcGIS applications?