The presented behaviour could appear mainly in two cases.
The first one is actually a bug in the IE browser which leads to the presented issue. Every Internet Explorer version (except IE10) on the other hand has the limit of loading only 31! stylesheets per file and thus fails to load all CSS files in a theme if they are more than 31. More information on this matter could be found in the blog post bellow:
You could try using RadStyleSheetManager on the page in order to combine these stylesheets into one request and see if this helps.
Another possible cause of the presented issue is in case you are showing the RadDatePicker control with ajax request. In case you have ajax on the page I would suggest you to disable it and verify if the same behaviour appear. If this is the issue you could use again RadStyleSheetManager on the page and register the Calendar component as a skinnable control like this:
sender, EventArgs e)
I hope this helps. Do not hesitate to contact us back if further assiatance is needed.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed