There shouldn't be issues with the same page if it worked before. There are some known issues related to the Browser Link feature in VS2013 so if you are using this version please disable the feature to see if this helps.
If it does not, I advise that you open a support ticket and send us a runnable sample where we can observe this behavior. The only other idea I can offer is to make sure the references to the assemblies are properly updated and the caches (ASP, VS) are cleared:
You can find a detailed list of the differences by going through the release notes for the versions in between: http://www.telerik.com/support/whats-new/aspnet-ajax/release-history.
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