I have same problem, umbraco was broken on my presentation for clients, it was stoped working with errors. I have demonstrate fronthend pages this was working well, then i have go to backend and i have change some text and publish, after that site was broken and fronthend pages not shown but errors. I was stop demonstration.

In umbraco log ther was error report:

At / (Referred by: ): System.FormatException: Unrecognized Guid format. at System.Guid.GuidResult.SetFailure(ParseFailureKind failure, String failureMessageID, Object failureMessageFormatArgument, String failureArgumentName, Exception innerException) at System.Guid.TryParseGuid(String g, GuidStyles flags, GuidResult& result) at System.Guid..ctor(String g) at umbraco.presentation.UmbracoContext.GetXml() at umbraco.UmbracoDefault.Page_PreInit(Object sender, EventArgs e) at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) at System.EventHandler.Invoke(Object sender, EventArgs e) at System.Web.UI.Page.OnPreInit(EventArgs e) at System.Web.UI.Page.PerformPreInit() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Version is 4.7.2 , iis 7, asp 4

I think i have found problem, but no right solution. in some cases and computers after publish and clicking on preview button, then error is show and site is not working, because UMB_PREVIEW cookie has no value. After deleting cookies site is working. 

If ia try again publish and preview, same thing, site was broken, i check cookie has no value ...

Tested on 3 computers

1. Desktop pc / WIN7 / Google Chrome new version - there is no any error, it is working well

2. Notebook / XP / Google Chrome new version - here is always site broken after preview ( UMB_PREVIEW cookie has no value. )

3. Client pc / i dont know whati he is using but same problem, after previw site broken ...

Please what can i do, is it ther any settings for disabling preview mode and not creating UMB_PREVIEW cookie or any other solution ?

Thank You