To reproduce:
-add a RadTimePicker;
-change its culture:
this.radTimePicker1.Culture = new System.Globalization.CultureInfo("en-GB");

Workaround:
public Form1()
{
    InitializeComponent();

    this.radTimePicker1.Culture = new System.Globalization.CultureInfo("en-GB");
    this.radTimePicker1.TimePickerElement.PopupForm.Height =350;
   
}