Windows Desktop background is not shown on some extended displays on Windows 7
This can become more prevalent when using multiple high resolution displays, eg UHD (4K).
This problem is specific to Windows 7 and earlier. This happens when the Windows desktop exceeds 8192 pixels in any direction. This problem is independent of DisplayLink software and can happen with directly connected displays. More information can be found in this Microsoft KB article.
To work around this problem, use the Screen Resolution control panel to rearrange the monitors so that neither the total width, nor the total height, of the extended desktop exceeds 8192 pixels.