I was able to reproduce the described behavior. The reason for it is that when the RadPane is initially collapsed, its sizes are calculated as for a collapsed pane - e.g its height/width is 0px. After that, when you expand it, the content url is actually loaded, but it is placed in the content wrapper DIV element of the RadPane and therefore it also has its size set to 0px and it is not visible.
For your convenience I prepared and attached a sample demo project which shows the desired functionality by using CSS.
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