I just upgraded from 5.0 (no SP1) to 5.1. When I try to open a frontend link without logged in, it shows "Object reference not set to an instance of an object." error message as shown below, But once logged in, I can open any frontend/backend links with no issues.  Here is the error,

Server Error in '/' Application.

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Object reference not set to an instance of an object.

[NullReferenceException: Object reference not set to an instance of an object.]
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.SelfAndBaseTypes(Type type) +9
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.FindMethod(Type type, String methodName, Boolean staticAccess, Expression[] args, MethodBase& method) +63
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.ParseMemberAccess(Type type, Expression instance) +508
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.ParsePrimary() +43
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.ParseUnary() +246
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.ParseMultiplicative() +26
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.ParseAdditive() +29
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.ParseComparison() +29
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.ParseLogicalAnd() +26
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.ParseLogicalOr() +26
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.ParseExpression() +23
   Telerik.Sitefinity.Data.Linq.Dynamic.ExpressionParser.Parse(Type resultType) +26
   Telerik.Sitefinity.Data.Linq.Dynamic.DynamicExpression.ParseLambda(ParameterExpression[] parameters, Type resultType, String expression, Object[] values) +47
   Telerik.Sitefinity.Data.Linq.Dynamic.DynamicExpression.ParseLambda(Type itType, Type resultType, String expression, Object[] values) +76
   Telerik.Sitefinity.Data.Linq.Dynamic.DynamicQueryable.Where(IQueryable source, String predicate, Object[] values) +62
   Telerik.Sitefinity.Data.Linq.Dynamic.DynamicQueryable.Where(IQueryable`1 source, String predicate, Object[] values) +58
   Telerik.Sitefinity.DynamicModules.Web.UI.Frontend.DynamicContentViewMaster.GetNonPagedDataSource() +505
   Telerik.Sitefinity.DynamicModules.Web.UI.Frontend.DynamicContentViewMaster.get_DataSource() +39
   Telerik.Sitefinity.DynamicModules.Web.UI.Frontend.DynamicContentViewMaster.InitializeControls(GenericContainer container) +26
   Telerik.Sitefinity.Web.UI.SimpleView.CreateChildControls() +58
   System.Web.UI.Control.EnsureChildControls() +129
   System.Web.UI.Control.PreRenderRecursiveInternal() +70
   System.Web.UI.Control.PreRenderRecursiveInternal() +216
   System.Web.UI.Control.PreRenderRecursiveInternal() +216
   System.Web.UI.Control.PreRenderRecursiveInternal() +216
   System.Web.UI.Control.PreRenderRecursiveInternal() +216
   System.Web.UI.Control.PreRenderRecursiveInternal() +216
   System.Web.UI.Control.PreRenderRecursiveInternal() +216
   System.Web.UI.Control.PreRenderRecursiveInternal() +216
   System.Web.UI.Control.PreRenderRecursiveInternal() +216
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2834

Try this, worked for me:

"It turns out that this is a bug and I have logged PITS issue so that you can vote for it and track its progress. We will try to fix it for the next release but for now I can suggest several workarounds. One is to set the view permissions in you content type (Content menu -> Module,  in the sidebar there is a link "permissions") to everyone. The other option is to display all of the items in your widget as it turns out filtering by categories and tags is causing the error. Unfortunately view permissions are not handled properly when in security config it is set to filter by view permissions. If you set the property filterQueriesByViewPermissions to false everyone will see your items and you won't be experiencing such problem. Otherwise consider the workarounds I have suggested."

We have this same problem after upgrading to  Sitefinity 5.1.3210.0 SE.
This is a huge problem!

Also after upgrading we can't reference a file on the root with an .aspx extension that we use to get and read an RSS feed.

Hi,
This error is coming on all the inner pages of the site. Index page is working fine. Please help me.
Server Error in '/' Application.

Object reference not set to an instance of an object.

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.NullReferenceException: Object reference not set to an instance of an object.

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:

[NullReferenceException: Object reference not set to an instance of an object.]
SitefinityWebApp.Custom.LeftMenu.LeftMenu.GetFirstNode(SiteMapNode currentNode) +56
SitefinityWebApp.Custom.LeftMenu.LeftMenu.GetFirstNode(PageSiteNode currentNode) +243
SitefinityWebApp.Custom.LeftMenu.LeftMenu.BindMenu() +350
SitefinityWebApp.Custom.LeftMenu.LeftMenu.Page_Load(Object sender, EventArgs e) +136
System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14
System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35
System.Web.UI.Control.OnLoad(EventArgs e) +91
System.Web.UI.Control.LoadRecursive() +74
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Control.LoadRecursive() +146
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2207