Visual Studio Editor does not underline errors anymore

My Visual Studio (2008) Editor has stopped to underline Errors (this nifty wavy red lines). I can't really tell when, but it can be related to the installation of .Net Framework 3.5 SP 1 or the MVC Beta (which I guess is unlikely). Furthermore have I installed and uninstalled both CodeRush and Resharper for evaluation purposes (decided not to keep either one of them).

Does anyone know the problem and how to restore this functionality again?

Have you checked Tools→Options...→Text Editor→C#→Advanced→Underline errors in the editor?

I usually like to reset my settings after messing around with plugins, as they tend to mess with settings: Tools→Import and Export Settings...→Reset all settings.

About possible causes.

For VS 2012 and 2013 if you have more than one instance of Visual Studio on different machines binded to one "live" account and have installed ReSharper on one of them, it disables the native IntelliSense and error underlines (to replace by it's own rules) that will be synchronised through your account to another machine without ReSharper.

Found it in Visual Studio 2019 as: Tools > Options > Text Editor > General > Show error squiggles

This is generally called Disable Squiggly or Wavy lines in Visual Studio.

How you will do in Visual studio 2013?

TOOLS -> Options... -> Text Editor -> C/C++ -> Advanced -> Disable Squiggles: True/False (Under IntelliSense) -> Press OK