I'm getting internal server 500 error when using the latest dotless DLL with a .NET 4.5 integrated or classic application pool. I currently don't have the 4.0 app pool (installed VS 2012 directly... so is this a 4.5 issue, or is there anothe way to make this work?
Switching to 4.5 integrated mode solved the problem, as well as making sure all LESS files are present, which I was missing one. Classic was not working for me.