We identified the issue and released a hotfix.
Please contact Quest Customer Support and ask for the hotfix. Please mention the hotfix for issue #184183.
If you have any problems getting the hotfix please let me know.
QWP 5.6 for SP2010 use Telerik controls version Q22011. Telerik has changed the skin folder structure from the previous versions.
In order to create a custom skin I'd recommend to get an existing Telerik skin, rename it and then perform your changes on that skin.
For example take Windows7 skin, make a copy of this folder and rename it to Windows8. Then modify the css files and/or the images to get your custom skin. To migrate your existing custom skin you need to follow the new folder strucure.
Please let me know how goes everything.