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‘Sys’ is undefined or ASP.NET Ajax client-side framework failed to load in a ASP .NET 4.0 website

‘Sys’ is undefined or ASP.NET Ajax client-side framework failed to load in a ASP .NET 4.0 website

I was getting the the error [‘Sys’ is undefined] and [ASP.NET Ajax client-side framework failed to load] in an ASP .NET 4.0 website. After reading and following the post [] I found my solution.

De ScriptResource.axd [long url] was throwing a 404 server error. This was caused by the fact that the website had an application pool set to .NET 4.0 but the [Handler Mappings] (see Screen dump 001) for the *.axd files where not registered. Even after executing the [aspnet_regiis –i –enable] the [Handler Mappings] where not registered. After re-creating the site, the error was resolved. The screen dump 002 shows the correct handler mappings.


Screen dump 001



Screen dump 002



  1. Man, you are a lifesaver. I had some downtime yesterday so I thought, “hey, why not move all of my company’s sites from a Win2003 server to a Win2008 server?” Long story short, it’s been a sucky 24 hours. I literally tried everything I found on the web: reinstalling the .NET frameworks, running aspnet_regiis –i, etc. and it was the handler mappings all along.

    Just in case anyone else comes across my comment, here’s what the root of my problem was and my particular solution. We have Plesk running on Win2008 on a dedicated server housed at a really big hosting company. When you create sites using Plesk, it apparently does not inherit the mappings from the parent. When I saw your sceenshot, I realized that the handful that were showing up on my plesk-generated sites could not be correct so I hit “Revert to Parent” on those sites and suddenly the nightmare was over and the errors went away. At least it’s over until I find out what the fallout is from removing the plesk-generated handler mappings.

    Thanks again.

  2. Thanks for the screenshots, they helped me to check whether everything is registered correctly. However, I solved the problem differently by using a suggestion found on another site: I repaired the “Microsoft .NET Framework 4 Client Profile”, and after rebooting the server the errors were gone (Windows Server 2008 R2 SP1, June 2010 Security Patches for the .NET Framework applied).

  3. I ran into this issue while developing in Visual Web Developer Express [visual studio (search term)], under Windows Azure simulation mode and discovered I was able to fix it by removing all Windows Azure Roles and re-adding them. Before I discovered that, I found I could solve the problem by recreating the application, which was a big hassle. To “recreate” the application I would delete the solution file and then manually add all the folders and then Add Existing files to populate them. Is there an easier way to recreate? Info: running W7 Home Premium.

  4. OK, skip my remove-the-role solution. This problem has occurred spontaneously three times now, and the current time the fact that I have removed and re-added a role has not made a difference.

  5. Although by removing the suo file and doing the remove and re-add the Web role just worked for me now. All my issues are in the development environment. Again, I am in a simulated Azure environment.

  6. Hello,

    I am using 4.0 and Ajax v4.0. I am using AJAXToolKit for quick search functionality..I am getting ‘Sys’ is undefined or ASP.NET Ajax client-side framework failed to load in a ASP .NET 4.0 website..When i run my appliction on local server then everything work fine but when i access my application from NLB client server then i get this error..I check both application server IIS setting, Machin.config, Web.config..all are identical..even i try to make changes in web.config file like putting handlers, etc.. but no luck. i assume in .net4.0 there no need to change in web.config as it is already taken care by Visual Studio…i gone through many blogs and tried all combination but no luck..:(..even i restall my framework on both application..rebooted many times…pls let me know if you can help response is appriciated..thanks


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