Solution (Thanks to tuxedo25 )
It sometimes happens that some files "are being used by another process".
Close your solution and delete all "bin" and "obj" subfolders of all your projects that are included in the solution.
Then open your solution again, execute "Clean solution" and build it again.
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My own solution: If the above has failed, sometimes, it seems that the compiler is storing some cache in the cloud or something. So, by disabling the network adapter and trying to compile again has proven quite effective so far. But it may still be just a slight coincident as I notice it failed a couple times sometimes.