Interesting that the log shows it compiles to 666 bytes even with the errors. I suspect that vm needs to halt the link processes when it encounters errors.

Anyway that's an aside. Please delete the following temp folder and try the compile again to see if you still get the problem 

C:\Users\User\AppData\Local\VMicro\Arduino\Builds\Project\mega2560

Hopefully v28 will use some other way to uniquely store temp files for a project. 

The temp files are cached to help speed up compilation but in the case of duplicate project names it can end up with this problem sometimes. It seems that the current version sometimes gets confused when you create two projects of the same name. In your case you had a project called "Project" which is very generic name and know doubt, for testing, you has used the name for another project.

The next version (28) includes support for the Visual Studio "Solution Clean" features. This does the same thing as clearing/deleting the compiler cache in the temp folder and saves you having to do it manually. BUT, when we get time to change things this feature won't be needed

Sorry you hit the issue, thanks for persisting.