Sample CSS to work around the issue:

• Remove the horizontal line:

  html body .RadGrid.RadGrid_Windows7 .rgGroupHeader,
  html body .RadGrid.RadGrid_Vista .rgGroupHeader {
      background-image: none;
  }

• (optional) If you prefer to have visible delimiter below the Group headers you can use:

  html body .RadGrid.RadGrid_Windows7 .rgGroupHeader > td:last-child {
      border-bottom: 1px solid rgb(155, 201, 255);
  }
  html body .RadGrid.RadGrid_Vista .rgGroupHeader > td:last-child {
      border-bottom: 1px solid rgb(146, 152, 161);
  }

  *colors in the snippet above are exemplary

Kind regards,
Doncho
Progress Telerik