The latest AppBuilder Extension for Visual Studio, v2015.3.805.2, seems to be blocked by Windows Defender.
Steps in order to reproduce the problem:
1. Update the extension through Visual Studio
2. Restart Visual Studio and open a previously working solution
3. A dialog appears, stating that the project could not be opened due to an incompatible version
What actually happened in the background is that Windows Defender seems to quarantine the Telerik.BlackDragon.Common.dll, located in the %APPDATA% folder, as it considers it to be a "VirTool:Win32/VBInject" virus infection. See the attached screenshot for more details.
This was reproducible not only on my system, but on a colleagues' as well. Both are running Windows 8.1, and we're running VS2015 and VS2013 respectively.
Current workaround seems to be to restore the quarantined items through Windows Defender and all will work well again. Assuming this is indeed a false positive.