Could you clear the logs in AppData folder of the website, then try to replicate the issue and see if there is something logged in. It looks like the error is related to a custom template you have or a template that was modified. The service used to get the templates fails.
You can also enable service trace logging
to see if it will provide more details.
I am sending you a sample code in the attachment used by the service that you can run against the website in debug mode and see if there will be an issue thrown by a method, so you can see the actual exception and try to narrow down if there is a problematic template. You have to add the WebForm1.aspx from the attachment to the root of the website, build the site and attach a break point to GetControlTemplatesInternal method which retrieves the templates.
I checked this on a new and empty website( using build 4.3.1885 -official 4.3 Release) and templates loaded correctly. I suppose that there is something at your website that breaks the response of the service used to get the templates.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items