After making sure my Visual Studio always ran as an administrator by following my own post HowTo: Have Visual Studio always run as administrator on Windows 8, I got a message if I wanted to save changes to devenv.sln each and every time I double clicked a Solution. Since the problem kept occuring after installing Visual Studio 2015 CTP6, I wanted to solve it. So here we go!
Double clicking a Solution file doesn’t start devenv.exe (which is set to be run as an administrator) but will cause the Microsoft Visual Studio Version Selector (VSLauncher.exe) to be run. This tool selects the right version of Visual Studio to open the Solution. Because VSLauncher.exe is not being run as an administrator, the error occurs.
Find VSLauncher.exe, which is typically located at ‘C:\Program Files (x86)\Common Files\Microsoft Shared\MSEnv’ and make sure it runs as an administrator too. This will cause the Version Selector to be started with the correct privileges, helping it start devenv.exe with the right privileges. Problem solved ;)
Hope this helps