Keep IIS express running
It may happen that the IIS express will close when the debug mode is turned off. This is a setting that can be changed on the project. Simply turn off "Enable edit and Continue" option, shown in picture below.
Problem with MetaData File not found .dll
"Metadata file 'C:\Projects\XYZ.dll' could not be found"
If you check the "output" tab instead, sometimes it give you extra information, like mine:
could not be resolved because it was built against the ".NETFramework,Version=v4.5.2" framework. This is a higher version than the currently targeted framework ".NETFramework,Version=v4.5.1".
Make sure that your project depends on the same version of .net, and change it to the lower one if needed.