It seems to be caused by trying to view a logged error. When I click on the most recent error in my log, this exception is thrown:
System.InvalidOperationException: Error during serialization or deserialization using the JSON JavaScriptSerializer. The length of the string exceeds the value set on the maxJsonLength property.
at System.Web.Script.Serialization.JavaScriptSerializer.Serialize(Object obj, StringBuilder output, SerializationFormat serializationFormat)
at System.Web.Script.Serialization.JavaScriptSerializer.Serialize(Object obj, SerializationFormat serializationFormat)
at StackExchange.Exceptional.Error.ToDetailedJson() in C:\BuildAgent\work\d20fce4a5bb47bd3\StackExchange.Exceptional\Error.cs:line 500
at StackExchange.Exceptional.Pages.Master.Execute() in C:\BuildAgent\work\d20fce4a5bb47bd3\StackExchange.Exceptional\Pages\Master.generated.cs:line 0
at StackExchange.Exceptional.RazorPageBase.TransformText() in C:\BuildAgent\work\d20fce4a5bb47bd3\StackExchange.Exceptional\RazorPage.cs:line 57
at StackExchange.Exceptional.RazorPageBase.TransformText() in C:\BuildAgent\work\d20fce4a5bb47bd3\StackExchange.Exceptional\RazorPage.cs:line 57
at StackExchange.Exceptional.RazorPageBase.ProcessRequest() in C:\BuildAgent\work\d20fce4a5bb47bd3\StackExchange.Exceptional\RazorPage.cs:line 70
at StackExchange.Exceptional.RazorPageBase.System.Web.IHttpHandler.ProcessRequest(HttpContext context) in C:\BuildAgent\work\d20fce4a5bb47bd3\StackExchange.Exceptional\RazorPage.cs:line 68
The log entry in question probably has a large exception message (it contains logged SOAP communication). If you need a DB dump of the entry, I can provide it to you via private communication.