Orchard CMS: Strange random crashes of WebMatrix 3

Did a clean install of Orchard 1.8.1 via Web Matrix, all default settings. Web Matrix crashes when I open Modules/Orchard.Blog/Models/BlogPost.cs in the editor. Same for BlogPart.cs and other random files.

Error in application log:

Faulting application name: WebMatrix.exe, version: 7.1.1932.0, time stamp: 0x52589c06 Faulting module name: Microsoft.WebMatrix.ExceptionUtilities.ni.dll, version: 1.0.0.0, time stamp: 0x501aaffc Exception code: 0xc0000602 Fault offset: 0x00004f04 Faulting process id: 0x1128 Faulting application start time: 0x01d04cbc36ed11be Faulting application path: C:\Program Files (x86)\Microsoft WebMatrix\WebMatrix.exe Faulting module path: C:\windows\assembly\NativeImages_v4.0.30319_32\Microsoft.W95c4004b#\726122eea182115361e9330f6158417a\Microsoft.WebMatrix.ExceptionUtilities.ni.dll Report Id: 96431a7d-b8af-11e4-828d-600292c98618

My module code also fails if I include Orchard.Blogs/Models in my code. This is the bigger problem.

Has anyone seen this before?

WebMatrix is a dead project. It is not able to support anything that targets .NET 4.5 or higher. It is officially "no longer under active development" and features a number of long standing bugs. You should use Visual Studio 2013 Community Edition if you want a free tool for ASP.NET development.