This just happened to me as well also on DNN 4.8.4. Below is the complete "Yellow Sceen" Text. Restarting the App Pool got the site going again.
Server Error in '/' Application.
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Hashtable insert failed. Load factor too high.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.InvalidOperationException: Hashtable insert failed. Load factor too high.
Source Error:
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
Stack Trace:
[InvalidOperationException: Hashtable insert failed. Load factor too high.]
System.Collections.Hashtable.Insert(Object key, Object nvalue, Boolean add) +2903977
System.Collections.Hashtable.set_Item(Object key, Object value) +11
System.ComponentModel.ReflectTypeDescriptionProvider.ReflectGetAttributes(Type type) +323
System.ComponentModel.ReflectedTypeData.GetAttributes() +36
System.ComponentModel.DefaultTypeDescriptor.System.ComponentModel.ICustomTypeDescriptor.GetAttributes() +50
System.ComponentModel.TypeDescriptor.GetAttributes(Type componentType) +26
System.Web.UI.view_stateModeByIdAttribute.IsEnabled(Type type) +46
System.Web.UI.Control.Saveview_stateRecursive() +117
System.Web.UI.Page.SaveAllState() +466
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2357
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Version Information: Microsoft .NET Framework Version:2.0.50727.1433; ASP.NET Version:2.0.50727.1433