The version used is an old and it refers to the discontinued Classic Telerik controls for ASP.NET. These controls are not supported and also the .NET 2.0 is also not supported.
Also, I am unable to define exactly why this happens. As you described, this happens inconsistently which does not lead me to a possible configuration-related reason for that behavior.
However, I suggest you checking for any content filters related to Relative and Absolute URLs and see if disabling or enabling them would resolve the issue.
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