I get this too. Clean 4.0.2. 2003 server. 2000 SQL server.
[NullReferenceException: Object reference not set to an instance of an object.]
DotNetNuke.HttpModules.UrlRewriteModule.OnBeginRequest(Object s, EventArgs e) +702
System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +92
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +64
This error occurs randomly, though it seem to occur more frequently in daytime. Other things that may be interesting is that the server response time increase in the daytime than the evenings, which lead me to think it may be the server hardware? I will check this with my provider, fastname.com.
I also got a lot of DotNetNuke.Services.Localization.Localization and some Solpart.WebControls.SolpartMenu errors.
It should be established a bug report system for DNN where we could report in spesific errors when they occure. No problem solving, just reporting where we could include the error and DNN version and maybe server software and hardware. Simple as possible.
With kind regards,
Bonavox