I think that you won't be able to post back directly to block's controller. That's pretty tricky. Is it possible for you to have a AccountController for instance with Logout method?

If you are dealing with multiple languages in your site, remember to include language routing value (in this case - code fragment posts back to the same page):

Html.BeginForm(null, null, new { language = ContentLanguage.PreferredCulture.Name }, FormMethod.Post)

You can not go directly to a EPiServer block but you can go directly to a MVC-controller.
For exampel create a Controller like this:

public class LogoutController : Controller
{
	[HttpPost]
        public ActionResult Logout()
        {
            FormsAuthentication.SignOut();
            Session.Clear();
            UrlHelper url = new UrlHelper(System.Web.HttpContext.Current.Request.RequestContext);
            Redirect(UrlHelpers.PageLinkUrl(url, PageReference.StartPage).ToHtmlString());
        }
}

After that add a route to this like this:

routes.MapRoute("Logout",
                            "Logout/{action}",
                            new { controller = "Logout", action = "Logout" });

And change your beginform to this:

@using (Html.BeginForm("Logout", "Logout", new { language = ContentLanguage.PreferredCulture.Name },FormMethod.Post))
    {
        
    }

Please be aware of typeó since I have not tested this, but it should work

Yes, that worked! The problem was the BlockController as you said.

I added the route and removed the block, decided to use the button in my layout file instead - less hassle.

Thank you for the quick answers Valdis and Henrik!

Regards,
Ludvig Flemmich

Glad that it worked!

Your welcome

It is actually possible to post directly to a BlockController, although I would not recommend it if you have other options. By defining a custom route (e.g. in Global.asax) you can target your block controller like this:

routes.MapRoute(
    "CustomLogoutRoute",
    "logout",
    new { controller = "LogoutBlock", action = "Logout" }
);