I have an older (4.0.2.1) umbraco site that has a periodic problem where the users suddently can not login to the backend.  Restarting the w3svc service seems to solve the problem.

There seems to be a connection to these events being logged on the server:

Log Name:      Application
Source:        ASP.NET 2.0.50727.0
Date:          24-01-2012 12:01:35
Event ID:      1309
Task Category: Web Event
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:     xxxxxxx
Description:
Event code: 3005
Event message: An unhandled exception has occurred.
Event time: 24-01-2012 12:01:35
Event time (UTC): 24-01-2012 11:01:35
Event ID: 1d5ccc9d4ee44b6896ebf1457f0bf1d2
Event sequence: 1280
Event occurrence: 1
Event detail code: 0
 
Application information:
    Application domain: /LM/W3SVC/1/ROOT-1-129718548513140000
    Trust level: Full
    Application Virtual Path: /
    Application Path: C:\inetpub\wwwroot\
    Machine name: xxxxxx
 
Process information:
    Process ID: 4140
    Process name: w3wp.exe
    Account name: NT AUTHORITY\NETWORK SERVICE
 
Exception information:
    Exception type: TypeInitializationException
    Exception message: The type initializer for 'umbraco.BusinessLogic.Actions.Action' threw an exception.
 
Request information:
    Request URL: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
    Request path: /umbraco/umbraco.aspx
    User host address: 213.173.231.18
    User: 
    Is authenticated: False
    Authentication Type: 
    Thread account name: NT AUTHORITY\NETWORK SERVICE
 
Thread information:
    Thread ID: 3
    Thread account name: NT AUTHORITY\NETWORK SERVICE
    Is impersonating: False
    Stack trace:    at umbraco.BusinessLogic.Actions.Action.GetJavaScriptFileReferences()
   at umbraco.cms.presentation._umbraco.RenderActionJS()
   at umbraco.cms.presentation._umbraco.Page_Load(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.Web.UI.Control.OnLoad(EventArgs e)
   at umbraco.BasePages.BasePage.OnLoad(EventArgs e)
   at System.Web.UI.Control.LoadRecursive()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Does anybody know what causes this and how to fix it ?

Thanks

Mikael