This is related to a new feature in ASP.NET 4.0 for supporting extensionless URLs on IIS 6.0.
Normally, you should never see the URLs ending with "eurl.axd/xxx".
1. (Not likely your case) After ASP.NET 4 has been enabled on IIS 6, ASP.NET 2.0 applications that run on IIS 6 might generate this error.
2. A conflicting 3rd party ISAPI filter (like a rewrite filter)
3. (Likely your case) A redirection you setup in IIS. For instance, redirecting non-www records on a website to another in the same pool.
1. Use the registry key to just turn this ASP.NET 4.0 feature off. HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ASP.NET\4.0.30319.0 > DWORD EnableExtensionlessUrls to 0 , then restart IIS.
2. Install your ISAPI filter URL rewriter at the global level at a priority higher than aspnet_filter.dll or insert a rule in your rewriter to completely ignore URLs with eurl.axd in them. This might be as simple as
RewriteRule eurl\.axd -
References:ASP.NET 2.0 Applications Might Generate HttpException Errors that Reference eurl.axdHow to Disable the ASP.NET v4.0 Extensionless URL feature on IIS 6.0ASP.NET MVC eurl.axd errors