An update; I got it to work by changing from msbuild to build script as follows: msbuild "Src\TW.CommentSubsystem.Core.sln" /verbosity:minimal /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
Maybe the msbuild task ise hardcoded to use version 12?
I deleted/recreated the .sln inside visual studio 2015 and that resulted in a .sln file looking like "Microsoft Visual Studio Solution File, Format Version 12.00
# Visual Studio 14
VisualStudioVersion = 14.0.22512.0
MinimumVisualStudioVersion = 10.0.40219.1 [...]"
after checking this in it still was built with msbuild 12
Just for fun I also tried manually editing the "Format Version" to 14 but no cigar either.