Thank you for the detailed explanation. I managed to reproduce the described behavior. After a further investigation it turned out to be connected with the moment the visibility state of the Grid is set when the FileExplorer's Explorer mode is configured explicitly. I have logged this behavior as a bug which progress can be tracked here: feedback item
Basically, moving the Grid-related logic to the Page PreRenderComplete event handler will fix the issue, as the Grid will be made visible at this point:
protected void Page_PreRenderComplete(object sender, EventArgs e)
RadFileExplorer1.Grid.ClientSettings.AllowDragToGroup = true;
I have also updated your Telerik points as a small token of gratitude for the reported issue.
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