Actually, my scenario is a bit different but the results are similar.
I had just included several previously excluded projects back into my (VS2013) solution. They had references to an older version, 2014.3.1119.???, which caused build errors so I figured while I was at it, I'd upgrade the whole solution to the latest.
The upgrade wizard finished without errors.
However, the upgrade wizard always misses my Interface-only assemblies so I went back through them and changed the references manually.
I did see an unexpected mix of version numbers but once I finally resolved all of those errors, the build was still failing with access issues:
Error 13 Could not copy "obj\Debug\FarmingtonCo.Common.Interfaces.dll" to "bin\Debug\FarmingtonCo.Common.Interfaces.dll". Exceeded retry count of 10. Failed. FarmingtonCo.Common.Interfaces
Error 14 Unable to copy file "obj\Debug\FarmingtonCo.Common.Interfaces.dll" to "bin\Debug\FarmingtonCo.Common.Interfaces.dll". The requested operation cannot be performed on a file with a user-mapped section open.
When I try to maually delete the target file in the bin\debug folder, I see the error:
Error deleting file 'FarmingtonCo.Common.Interfaces.dll'. The system call level is not correct.