To workaround this issue you should dispose the Instance property of RadMessageBox after each show of message box. For example:
WORKAROUND:
RadMessageBox.Show("Text");
RadMessageBox.Instance.Dispose();
If you are using themes, consider the following approach:
RadMessageBox.SetThemeName("Windows7");
RadMessageBox.Show("This is some long text that sometimes does not wrap as it should.");
var field = typeof(RadMessageBox).GetField("radMessageBoxForm", BindingFlags.NonPublic | BindingFlags.Static);
field.SetValue(null, null);