This week, I installed Visual Studio 2015 in my laptop. Then all of websites I build which is running .Net Framework 4.5 or 4.5.x are not working. They got .net framework error Error 255 and it cannot execute vbc.exe. I did a lot of research. There are a lot of people wrote about this error is about App Pool Identity. They are right in the past, I found out if you have .net framework 4.5 an 4.6, this error is caused by some security patched, 3098779 and 3097997. If you uninstall these two patches, then you will be fine again.