The version you use is very old and does not support the current browsers (Chrome, Firefox, IE9 and above) and thus, I strongly advise that you upgrade to the latest.
The behavior your report is not a known problem with the current version and you can test our demos to see how the control behaves: http://demos.telerik.com/aspnet-ajax/scheduler/examples/overview/defaultcs.aspx.
Also, IE10 and IE11 have known issues with MS AJAX in .NET 4.0 and you can read more about this in the following post: http://www.telerik.com/blogs/how-to-get-your-asp.net-application-working-in-ie11.
If updating to the latest version and applying the information from the blog post do not help, I advise that you open a support ticket and send us a small runnable sample where we can observe this problem so we can advise further.
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